Kim- First, congrats on the some good running- only missing 2 days in 6 months is amazing. And on signing up for TOU- it's a good race (I'm biased, since I live in Logan). But I have to warn you that this may be a long reply.
Andy and Dave gave some good advice. I definitely agree with Dave that you shouldn't be afraid to do most of your runs at a slow pace- 1 to 2 min slower than marathon pace. And like Andy says, the most important thing is to run consistent mileage, and to slowly keep increasing it if you can handle it.
I would not recommend cutting mileage at this point just to add speedwork. For one thing, your marathon is still a long ways away, and mileage will help more than speedwork. If you want to do a bit of speedwork, I would suggest doing a medium distance tempo (maybe do 5-6 miles at MP as part of a 10 mile run, then increase the tempo distance and run distance as you get better), but make sure to give yourself enough easy runs before and after.
2 other things I noticed that may help you. First, I think part of the reason your long runs are ending terribly is cause you probably aren't fueling sufficiently (based on a few comments about "stopping for water" on your long run entries). On runs over 1 hour, and especially 2 hours, you need to make sure you drink a fair amount and even take some calories (gatorade or gu or whatever). This would be even more important in a hot, humid environment such as Mexico. Stopping at a drinking fountain once or twice during a long run is not sufficient. I would suggest either carrying a water bottle and drinking ~20 oz per hour, or stashing some bottles along the way. Even very experienced marathoners start bonking around 2-2.5 hours if they aren't fueling. Plus you need to practice this for TOU marathon.
The second thing that may help you is to stop racing your long runs! It seems like you do ~1:50 for the first 13.1 of almost all your long runs, then fall apart. Very few of your long runs should be at marathon pace- they should be slower. Sometimes much slower. It is more important to get in all the miles without dying than it is to push hard the first part, then limp home. If you want to do a harder long run, then I would do the first 12 miles at a slow pace (9:30 min/miles or slower), then increase your pace for the last miles. This will enable you to run farther and teach your legs to run fast at the end of a long run. But I would suggest that you do some long runs WITHOUT a watch! Run a known distance, but run relaxed and by feel rather than trying to break 1:50 each week (i.e. it worries me that you track your 13.1 splits each week). I think your weekly very hard long runs are wearing you out! Give yourself a time trial once a month (max) if you want, but stop trying to outdo yourself each week. Just trust that you are getting fitter, don't try to prove it. I would even suggest trying a run-walk cycle for your long runs- maybe run 8-9 min, walk 1-2, for the WHOLE run. Including the first mile. I guarantee you will run farther and faster at the end of your long run. Remember- you only BQ if you finish the whole marathon in 3:50... they don't give you any awards for doing the first half of you marathon in 1:50 and the second half in 2:20!
Just so you know, I rarely do many of my long runs (or any runs) at marathon pace. Doing a long run with 13 miles at MP is hard enough that I only do it once or twice in a 4-6 month period. Doing it every week is overkill. For a 3:50 marathon, McMillian calculator says your long runs should be 9:17-10:17 pace, and your recovery runs should be 10:17-10:47.
As for TOU specific advice- Train on some downhills so your legs can take a pounding. And do some of your long runs so the second half is on rolling terrain, like the TOU course.
This is a lot of advice, but you asked for it
. Hopefully at least a little of it is helpful. But I really think you can BQ, even in your first marathon. Keep increasing the miles, fuel properly, and slow down on your long runs. Best of luck!
