I agree with Southern that they are probably not very much use as a prediction tool. You are better off with calculating the equivalent time marathon time off of an underdistance race (HM or longer).

However, I do think MP is good training for the marathon. I run a midweek MP run of 13 miles by feel and keep track of the mile splits in my log. As the times come down, I believe it is a decent sign that I am getting more racing fit for the marathon. I think it might work as a negative indicator -- if you can't do 20-25k at your MP fairly easily, you might be in trouble trying to maintain that pace for an entire marathon even considering that you've tapered for the race.

I think shooting for 3 hours is how you might want to go out. Pick it up at mile 15-20 if you feel relaxed.

I would be concerned that you felt muscle soreness at mile 13. How's your diet? I would look at the foods that you're eating. Are they un-processed, fruits, veggies, spices and herbs?

Anyway, I don't think you can handle the pace if you feel soreness at mile 13 of a MP run.

Good luck!

ljwoodw touched on an important point for preparation: long runs. Better than your "25K @ MP predictor workout" would be ljwoodw's 22 miler with the final 10 at MP - can you this? With only 2 x 20 milers under your belt 4 weeks out, I would try to squeeze in a fast-finish 23-24 by 3 weeks out. Try ramping up to MP for the last 12 miles with minimal stoppage time for sport drink. It may not be a predictor, but at least better train you for that final 10K push.

Another suggestion is the 10x1 mile (2:00 rest) @ 10 K-10 mile race pace my training group does 12 days out. Your training appears to be sufficient to be able to execute and recover from this. It's a great workout that trains toughness and will make marathon pace seem like a massage.

Leon is right in his own way. But all that we look for are indicators of which pace we should attempt to make our race day MP. Reality is that MP would be different almost every day, week, month - Our bodies have something a little different as way of potential each day.

The accuracy of predictors or inaccuracey usually revolves around your mileage (aka General Aerobic Fitness) and the length of the race used as an indicator. A 1/2 marathon is a better predictor than a 5k. But a half marathon can be a lousey predictor at times.

2 local guys in my area run 1:17:46 and 1:17:47 - 1/2 marathon 3 weeks before the marathon we are all running. So I expect to see them around during the race or be behind them. My best 1/2 is a 1:22:18 (Way back in May). Marathon comes - I run a 2:45 and they run a 2:56 and a 3:05.

What was the difference - I sure the heck don't know - I only know me. Because I run lots-o-miles and what ever else I do - I am able to hold a pace longer even when fatigued.

So to answer the question - You take a bunch of data - 1) all you race times 2) All your workouts and 3) How you feel the day of the race and make your theory of what MP should be and then you test that theory.

With apologies to wanderingrunner for the combination shameless plug/hijack:

Firstly, you should know that my group precedes this Tues night workout (12 days out from race) with an 8x 1mile (2:00 rest), and a 7x 1mile (2:00 rest) workout one and two weeks prior to the 10x 1mile (2:00 rest), respectively. We run them on a well-calibrated pedestrian/bike path straightaway in a park with a gentle roll and slight hills at either end, as would be typically encountered in a road race. It is VERY helpful to do these with a friend or two who plans to complete ALL the reps.

For warmup, we jog a mile, stretch, then do three or four 50m strideouts. The cooldown is a half-full mile jog + stretching. The workout is always scheduled 2 days after the Sunday long run with a rest day in between. The following days (Wed) are just 5-6 miles easy.

It's a 10x1 mile (2:00 rest) aerobic threshold workout, so ideally you want what your 10 K pace would be on THAT day (with no taper), or round it up to what you think you are capable of producing for ALL the repeats. Your lactate threshold pace is supposed to be what you could only just hold for an hour, so you want to try to be faster than this by as much as possible. For example, my best recent half was 1:24:01 and I was shooting for a sub-3 hour marathon, so I did the 10x 1mile repeats in 6:00 each, plus/minus 2-3 seconds. On a track I would pitch for a little faster (5:50-5:55?).

As for long runs leading up to race day, we do a 20-22 miler three weeks out, followed by 16 and 12 the following weekends, respectively. All of these finish with some marathon pacing, the amount of which depending on your fitness.

Incidentally, I imagine the consensus on this forum would be that your slow marathon times (relative to 10K, half mary) might be due to a deficiency in quality long runs this training cycle (last 20 weeks-ish).