Marathon Training Help Required

What have you based your proposed marathon pace on? Previous result at the half marathon? How long have you been running? How many times/miles a week do you run and what do you do in each session?

You shouldn't be running your long runs at marathon pace - that's why they're known as long, slow, runs and treadie speed won't be the same as that you can sustain on the road. Run your long and recovery runs easy and work hard at your interval, tempo, hill sessions - whatever it is you're doing for speedwork.

I targeted 4hr based off a 1:55 HM in early training, managed a 1:46 half a month before, but ended up with 4:18 (~1:52 half way) - The main reason, looking back, was not enough miles (25-30/week) and not enough LSR's at 20 miles (4 over 18 miles, none over 19).  My increased pace over training came from longer slower runs, I didn't do any speedwork - probably dropped 1 min/mile over 4 months.

This time I'm building up (for Sept) with an aim of at least 5 x 20 miles LSRs, and probably around 45 miles a week.

I think a reasonable conversion without a solid endurance background is 2 x HM PB plus 20 - 30 minutes. It's those long runs that make the difference definitely. I rarely do speed work as I tend to break but have gradually brought my times down over all distances with consistent training. Stick with it, slow down the longer runs and get those 20 milers going and enjoy the ride.

I have my first marathon in three weeks, and am doing a long slow 20 mile one, but i just listen to my body without any speed in mind, other than to just take it easy. Average about 8.2 minute mile in the second 10 mile stretch, first 10, about 8 give or take. Bearing in mind i am a 7 minute miler normally and have run 15 half's. I think everyone is different therefore, but i do try and fit in an 8 miler at normal half marathon pace. No real speed work, but lots of stretching on days off, and maximum recovery after long run, to avoid injury.

I have been increasing my distance since January, which is why 20 miles is quite comfortable now, and feel no side effects the next day. Its all about pushing your body further, but not taking any shortcuts, and trying to get there quicker than your body is ready for.