I guess the first question is: are you quite, quite mad?

I can’t decide if I’m mad or just stubborn. It’s one of those things that I suggested, and everyone sort of went: “No, that can’t be done. It’s impossible.” And the more you hear that, the more you think: “Well, why can’t it be done?” We brought in a guy who’d done something similar, a health and safety guy and a fitness expert, and they were all like: “Nah! There’s this to consider, there’s that to consider…” It was so, so negative.

Who came up with the idea?

I did an ultra-marathon last year, so Sport Relief asked if I could do something for them. Someone said: “Why don’t you try to row or kayak or swim a bit of the Amazon in a way that no-one’s ever done?” And I said: “Well, why do a bit when you can do the whole thing?” That’s when everyone said: “Don’t be stupid!”

You’re planning to do 2010 miles. It’s only 874 miles from Land’s End to John O’ Groats.

I know! But it’s not just that, the width is massive as well. It’s 60 miles wide at some points. I thought on one hand that it’ll look good on telly, but on the other hand, it means if I go wrong by a little bit and veer right or left, it could take me hours to get back on track.

Well, I hope your sat nav works.

I know! But it hasn’t turned up yet!

Have you done any kayaking or rowing before?

I’ve never rowed and I’ve only been in a kayak once. When I joined Blue Peter, we went out whale watching in a kayak, for just one day. There wasn’t really that much time to train, either. By the time we got it signed off, it left us about six weeks. So the time was really precious, and someone said that before I thought about training, I needed to get in a kayak and see if I could actually do it. There is quite a technique, and if you don’t get it right it just takes ages to get anywhere. I’ve definitely found that, the more I’ve practised, I go further with the same amount of effort. I’m no expert, and I don’t look that graceful, but I’m getting there.

How have you been preparing?

When we got the nod, I was doing the New York Marathon, so my first bit of preparation was when I needed to start thinking about using an oar. I had to borrow a broom handle from the hotel reception ­– they thought I was bonkers! I lied and said that I needed it to stretch my shoulder muscle. So I borrowed this broom handle and sat on the end of my bed, doing a paddling motion. Every Sunday I’ve been going out on the water with a GB coach, then going to the gym and doing loads of weights. Which has been grim, to be honest. I love being on the water, but lifting weights and doing 300 push-ups is not fun.

What do you think your main problems are going to be?

Physically, I’m going to be exhausted, obviously. But mentally, it’s also going to be the same motion day after day, doing the same thing for 10-11 hours, for weeks. I’m quite a fidgety person and quite busy, so having to sit in one place for that amount of time is going to be really difficult. What I didn’t think of, is that when you’ve been in the boat for a few hours, your bum and your legs go numb and stiff. I’ll probably also get blisters on my hands and on my bum.

Have you been reading up on the creatures that live in the Amazon?

My mum and dad, bless them, are real worriers, and they’ve been trying not to ask me too much about this. I suppose that they think ignorance is bliss! But my dad came to me the other day, and he said: “So, bull sharks…you’ll pass quite a lot of them, won’t you?” When I said I would, he just sort of went white and shook his head. I don’t think piranhas are anywhere near as bad as people think. I am worried about the bull sharks, but also the monkeys, because they carry yellow fever. If you get that, you die. A friend of mine said: “Aww, just think of the animals you’re going to see. You’ll see monkeys, parrots…” I was like: “I don’t want to see monkeys!”

What will you be eating?

The amount of calories you have to consume just to keep going is phenomenal. I’m going to be eating on the boat, probably just a lot of potatoes and pasta. But the thing is, because I’ll be out there for seven weeks, everything has to be freeze-dried or long-life. Pot Noodles aren’t exactly what you’d imagine the food of an athlete to be, but that kind of thing’s perfect.

I nearly said it might be nice for you to get away from the cold weather, but I know that you like icy, snowy days.

Yeah! I love these clear, crisp days at the moment. But yeah, it’ll be nice to have a bit of heat. Apparently it’s 100% humidity though, so I’m expecting to have massive hair. I’ve had all my hair cut to shoulder length and had it thinned out, so I’m hoping that’s going to help me. There’ll be many a hat worn though, to cover up the bad hair days!

Do you always like pushing yourself physically?

I think I’m just stubborn. People say no and it makes we want to do it. People are already asking me what I’m going to do next. I keep working on Everest, but my boss is just like: “Helen, shut up!”

Why did you think it was important to do something for Sport Relief?

Partly because I’ve been to one of the projects, and it had a massive effect on me. I went to Folkestone to meet a young boy who’s a full time carer for his mum, and he went to a youth club once a week, just to get away. It was his once chance a week to be a kid. He did everything for his mum: he shopped for her, he cooked for her, he cleaned for her. I even said to her: “I really like your boots,” but she didn’t know where they were from, because her son had bought them. He’s 13! When I’m moaning about my challenge, I need a reason in my head to keep going, and I don’t feel like I can let Joshua down. I don’t feel like I’m in any position to moan, because: a) I’m in the Amazon, and b) I’m attempting something that I’m going to able to talk about for the rest of my life. But if I stop, I’m going to be letting him down. And also, you see a lot of women on the telly with big boobs and big hair, but do you see women on telly who are attempting something sporty, something difficult? I’m really lucky that a lot of readers get in touch from Blue Peter, and off the back of my ultra marathon, a lot of them joined a running club. And it’s not about running 78 miles, it’s about pushing yourself, and hopefully I’m going to be able to encourage girls – and boys –  to do that.

Well, the last thing to say is: good luck! You’re much braver than I am.

It’s not bravery, it’s stupidity!

Blue Peter hope Helen's incredible efforts will inspire kids across the UK to rise to their own challenge for Sport Relief. Go to sportrelief.com/milo now to find out how you can get involved.