@iswimibikeirun: The advice I’ve gotten, and am generally following, is a bit different. My training plan has long runs that are all even numbers. 6, 8, 10 and 12 miles runs. So I’m adding 2 miles to my long run each week. For example, this is what my training plan looks like (below). I’m not following it exactly, since my work and home schedule isn’t this consistent, and sometimes I just run when I can fit it in, but I’m following the long run schedule.
You can find a pace calculator here. If you enter the pace/time it took to run a certain distance, it’ll give you a predicted time/pace for a different distance. So my 4 mile race this weekend was 40 minutes, 10 seconds, so my half marathon time is predicted to be 2 hours and 20 minutes.
I ran mostly negative splits this weekend in my 4 mile race 10:08, 10:16, 10:08, 9:43, but especially in longer races it’s not as common for a newer runner to get negative splits. It’s hard to hold back enough early on, to have the extra energy later when you’re getting more and more tired. I wouldn’t worry to much about your splits, other than trying to keep them close to the same if you can.
Also my training plan has my longest run at 12 miles, which is the longest distance I’ll have ever run come race day. A lot of training plans I’ve seen have the long run about 10% shorter than the race distance, when it comes to half and full marathons. For me, I know that if i can run 10 miles, on race day I can push through to 13.1 miles, so if I make the 12 mile run, I’ll be in good shape. I think an 11-12 mile run before a half is adequate.
This is all based on my knowledge as am amateur, just what I’ve read and seen in the last few years that I’ve been running. There are probably better sources out there, a local running club, a trainer, etc…