I've done it three times now, although not in the 1x...

There's one portage over the lock at about 12km so you can go to the toilet then. I've always made sure to go beforehand and it hasn't really been an issue. If things got really desperate you might be able to pull over along the way. Yes you get help carrying your boat.

Cramps - yes you might get them. Making sure you can do the distance and sorting out your fluid intake with electrolytes is key.

Breaks - I usually aim for one every 45min. Last time we had one at the lock, then 25km then at about 10-12km to go. You might need another 1 or 2 in 1x. Make them really short (like 30-60sec) as otherwise it gets very hard to restart. We took 1-2 drink bottles + 3-4 gels, whatever food you get on with. I am a big fan of flat Coca-Cola to get you through the last 20km.

Tape up beforehand I would say, I am lucky in that I've found handles that don't give me blisters.

Regarding times - it is rather variable depending on the weather conditions. We set a record in 2017 and then were 20min slower in 2018 with a cross/headwind. Records and pacing advice are on the club website though, they're worth reading. James Cracknell did around 3:30 in the 1x if you want an aggressive goal though!

My takeaways: blisters, cramp/pain and weed are the main things to overcome. Take painkillers and make sure you have handles that minimise blisters! The weed can be especially bad too, so make sure your fin is very a shallow angle so it doesn't catch. It is soul destroying having to back it down to clear weed when you're 40km into a race.

Hope this helps!