Jegleston, the tri training will help with the overall cardio stuff but the best thing to do is to see how it works out. Best advice I have for you is to sort out a pace plan and stick to it. If you can run 85 easy then there is a temptation to go out to halfway in 85 but that wouldn't be a good plan. Even splits are unrealistic so you need to plan to slow a bit on the second half but not too much so I'd say something like target halfway in 90 (which is about 6% slower than you can do, so should feel easy). Then carry on at the same pace to 20 if possible, so hit 20 in about 2:17 or 2:18, so that leaves about 47 minutes for the last 6 or so at 7:40 pace.
Jegleston, double and add 10 to 15 minutes is a good guide so from 85, 3:05 is possible but you need to have done the miles so it is not a given. As SJ says, what sort of training and background do you have? And by the way, why did you do 85 comfortably when you could have gone harder and faster?
ceal, any plans to race last Friday of the month this year? I have done it a few times, Dachs has made a guest appearance and walked away with a medal, PC2 has unfinished business there.Once London is over I plan on doing a few in the summer.
Samir, Ealing Mile would be good. Next one is Friday April 4th and I'll be there, nice intensity during marathon taper. Tom Snow is supposed to be down: he ran 3:48 for 1500 aged 17 so you can chase him all day.
When running, I generally think about the finish. Say I start a half, by 3 miles in I ignore the fact that it is a half and treat it like a 10 miler, then 10k, 5k etc. This doesn't work for the marathon, need to get to halfway before you can even start to think about the finish.