Thanks chaps.
I meant to say it was 175m of elevation, it was the Swindon half, my local one. So thanks for the suggestions, I'll definitely try to find some events more local to here as a warm up. I'm looking to do a bit of cross country over the winter, though the distances aren't quite enough for the marathon prep but I guess will help with speed and strength.
I was struggling in the last 5km, and was starting to get over taken rather than overtaking. However, since I failed to slow myself to the 4:30/km pace for the first few, it kind of evened out to the pace I was after overall and the adrenalin got me through.
My 5km pace is under 4:00/km, so I think my endurance is letting me down at the moment and not achieving the times I possibly should as I increase the distances. 3:20 sounds like a decent target to set myself.
General rule of thumb is half time x 2 plus 20 mins for a marathon target if that helps. Obviously need to start building some longer runs into your programme, but don't neglect the speedwork - keep doing 5k runs and hard efforts up to an hour etc.