Marcus - if you've not done much over 7 recently, then perhaps you'll be better starting off with a 10. Pace wise, you will find it feels almost too slow, especially on the shorter runs, but once the runs are 15+ you'll be glad of the slower pace! I like to do lsr's on scenic undulating off road routes - I find it easier to switch off from the pace and enjoy just being out - I find it much harder doing that on a route where I usually hammer the pace.
ps. you don't necessarily have to increase the number of runs you do a week - you can train well for a marathon on 3 runs a week, as long as you do the right kind of sessions. On here theres a big spread of approaches - some find high mileage works, others do lower. In time you'll find what works for you!
YKW tomorrow, then great boo's up all day long. Gah.
Marcus - your key run sessions each week will be your long run, tempo run, and speedwork. Don't bin these. If you've got recovery runs in there - shorter slower runs - you can happily switch these for bike or swim sessions. You may find it helps you avoid injury by giving you a break between running days, and stops you getting bored of too much running.
What minni said about your long run - say if your recent lsrs have been 10, do no more than 11 or 12.
Marcus - welcome. Your PBs would indicate you've the potential for sub 3.15, but for your first marathon I think you are really sensible setting a more conservative target. You are more likely to have a good day out, and once you've done one you'll know how to improve on the next!
Sounds like you've not done runs beyond 13M before? so the most important thing will be to build up the long run distance gradually, get your body used to it so you avoid injury. Many on here have been doing this for years so can (relatively) happily bash out loads of 20's, but if you've not done this before you'll find that tough on your body. I think 7 20's is excessive - I'd say a go for minimum of 2 (that's the most I've ever managed!).
Also, I'm a massive fan of cross training - I think marathon schedules often have too much running. Long days out on the bike with friends are great for building your aerobic base.
Simon - your less is more comment made me snort out loud...