Few know the gruelling 26.2-mile London Marathon course better than Paula Craig, who has competed as both an able-bodied athlete and a wheelchair athlete.
The 41-year-old, who was runner-up in the wheelchar race last year, first took part in the London Marathon in 1995 but while she was training for a triathlon in May 2001, she suffered serious injuries which left her confined to a wheelchair.
But after only five months training, she went on to compete in the 2002 wheelchair race.
Here she tells BBC Sport about high-speed twists and turns of London's ultimate challenge.
START - BLACKHEATH
There is a lot less jockeying for position than people think. Mainly because there are fewer of us than the runners in the marathon.
At the start, you don't need to push so much as you just drift down the hill.
When I was running it was very much my race against the clock but in the wheelchair race it is all about drafting, or slipstreaming, a lot of the time.
Everyone tends to stay together in a pack and then at some point it's a case of whoever makes a break for it.
3 MILES - WOOLWICH
It is downhill until around Woolwich and then it levels out really.
There is a sharp left roundabout as you come down the hill. You don't even consider it when you're a runner, but when you are using wheels, it can be quite nerve-wracking because if you are not used to it, you can tip out.
For me, it is a case of whether I brake and risk losing the pack or close my eyes and try to get round it.
6 MILES - CUTTY SARK
I take quite a long time to warm up, so it would probably be around the Cutty Sark before I am feeling comfortably into it.
But as a runner, it takes less time - probably two or three miles - because it is downhill to begin with, although you are still using your legs.
There is a sharp bend at the Cutty Sark, too, but you are not going as fast because you are not going downhill, so you can push round there more comfortably and take it.
It is a 180-degree bend with an incline and cobbles so you get a bit of everything.
Your front wheel, which indicates which direction you are going in, can jump and that means you have to flick your chair to stay straight at the same time as pushing.
9 MILES - SURREY QUAYS
We don't notice much around Surrey Quays but it is a lot quieter there.
Around the course, when the crowds are there it's fabulous because you feel like everyone's cheering for you, but when they're not, you feel alone.
I think this is the case particularly as a runner because when you are pushing you are going a lot quicker.
12 MILES - TOWER BRIDGE
Then you turn into Tower Bridge and that is a lovely place to come into because there are masses of people and you don't even think of any pain you might have.
You know you're about halfway there and you've only got 13 miles to go. You're on your way home and I always found that a real boost.
It's also where racers tend to break away from the pack. Francesca Porcellato did that last year and went on to win the women's wheelchair race.
15 MILES - ISLE OF DOGS
After the excitement of the area around Tower Bridge, there is nothing special to see around here.
There is an underpass, which is not so nice, but you get through there quickly and there are usually plenty of crowds there.
When you spend months training on your own, you're just happy to be with people.
24 MILES - EMBANKMENT
As soon as you hit Embankment you feel like you're well and truly on the home stretch, regardless of whether you're running or pushing.
Up until this year, you got that feeling once you were over the cobbles at the Tower of London but the course has been changed this year so there are no cobbles to contend with there.
The bit I hated more than the cobbles was the back road afterwards with little speed bumps, which are horrible.
The speed bumps are also horrible when you run, too, because it just throws your pace a little bit.
When you get to Big Ben, that's it, you think you're sprinting.
You find the energy from somewhere, even if it feels like you have nothing left. You think to yourself - I have only got five or six minutes of effort left and I can do that.
To be honest - you're almost running on air at this point because the crowds are just incredible.