Sarah - well done today! That's a great effort and got you nicely up to 18 miles. I simply don't know how you do it on a TM. I wonder if its easier at the gym where there are no distractions? At home I struggle to do more than 6 or 7 miles and will only do that if there is absolutely no other alternative.
Tomorrow make sure you 3 miles is nice and slow.
I think you'd be better sticking to 18m all at an easy pace on Friday since you have quality on Tuesday and Friday. And perhaps outside?!! 
I did 17 on the hills in the snow yesterday, slow - over 10 m/m! My legs felt tired today so it was 4 miles avg pace 9:28. Not quite the same as racing a HM but I'm sure it'll payoff somewhere down the line.
Snow on again here so might have to do my reps on the dreaded TM tomorrow but will avoid it at all costs!
Does Hilly live close by? She would be a great running companion for your outdoor long runs at the weekend.
Gels - I don't normally start using them until the last few weeks. I use SIS and SIS only. I once overdosed on jelly babies, pre evening race after having nothing to eat all day, and found out before the end they didn't agree with me - had to dash into the forrest!
It was only a blooming 10k too!! After losing all my SiS gels within the first few miles of VLM last year I took a lucozade gel at mile 14 and cramps started by mile 18 and had to make a loo stop at mile 20. You live and learn. 