Marathon runners, what are your top tips?

I wish I'd kept my long runs a bit shorter, I don't think I'll ever go much above 16 in training again but injury prone. Had a stress fracture and missed the last six weeks of training (but did what I could to strengthen everything else) and had a really happy comfortable race on the day. Over time you figure out what works best for you.

I would add to check and double check stuff like where the start is and give yourself plenty of time, have done quite a lot of races but did a half today which was a shambles in terms of directions and picking up race numbers (among other things, it was advertised as an out and back mostly trail race but was 100% road, some of it multiple laps of a very busy dangerous road 😡). I've never had a DNF but if it had been a longer race I would have bailed. I don't cope well with traffic!

I wished that I had done nearer to 26 miles in training (only did 20).

The mental challenge was almost harder than the physical. Be prepared! Try not to walk as starting to run again hurts like hell.

Take some kind of painkiller before the race and have some to take during the race also. I used ibuprofen, but paracetamol would work.

Don't try anything different on the day. I felt nauseous as hell taking gels even though I'd used them in training runs; I wouldn't use them again. Too much sugar will make you nauseous; try taking some flapjack or similar as well as jelly babies.

Salt tablets are amazing! I wish I'd known about them when I ran my marathon. Take one before hand and it will help prevent cramp (I use ones from MyProtein).

Compression socks / leggings after long runs and sitting in a cold bath really help.

When I started to struggle I started talking to people around me which really helped. Don't be afraid to make temporary friends! And good luck!

Well done you!!

I got cramp at mile 18, needed salt! A lady appeared with some mini sausages, they saved me!!!

So don't go overboard on the jelly babies!

Interval training is far more helpful than lots of long runs.
Find a glucose gel that you like and stick with it. Some taste revolting and others give you the shits.
Get any aches/pains/niggles looked at early on so they don't escalate.

Also if you look on the London marathon website training section they have blog posts from Martin Yelling full of good advice.

Agree with above!
Training: good to have an extra week in case of a bad week. Be flexible, missing a run because you are too tired or unwell is ok, few people do every run on their plan.
On the day: be proud of yourself just for getting there and surviving all that training! If you're using gels or whatever, take some extra, I lost track of when I had had what and used more than I had planned too! If the weather is hot drink more.
I had a rough time in mind but my main goal was to appreciate the day, the crowds, the sights, rather than push too hard for a time. I came in a little slower than planned thanks to a misbehaving hamstring, but still had an amazing day.
Things may not go to plan on the day, try to stay calm and positive and whatever the result be proud of what you have achieved.
Good luck!

Keep moving!

Marathons are run mostly in the mind, remind yourself you can and you will if your mind starts to play horrible tricks on you.

Don't try ANYTHING new on the day, use only tried and long run tested kit including underwear and socks, perfect your hydration and fuelling strategies on training runs and don't deviate.