Tough run ... (2)

This morning, I ran a 20-miler, the last long run of a 16-week marathon training plan (Intermediate-High Mileage from the Run Your Boston Qualifier community). The run was a totally drenching one--and not from any rain! It was a tough one. I'm not sure if it was just the need to acclimatize to the higher temperature and humidity of summer or not bringing enough water, but I'd to struggle to get to the end of Mile 20. My plan was to run it as 4 segments (6.5, 5.5, 4.5, and 3.5), with 3 water-walking stops (2-3 min. @). The first 2 segments went fine, But, after Mile 13, at the end of a hill, I just felt I had to stop (not sure if the legs felt the same). I ended up walking a full mile, all the while debating with myself whether to take the direct route home and thus cutting the run to 16 miles or run the planned route and walk as needed. This being the last long run, I decided on the latter. I ran to 16.5, walked another 0.5, before running the rest of the way home. That second half of the run was almost entirely mental. It was as interesting as it was frustrating. Last year, on an early summer 10-mile run, I had a similarly tough run. Anyways, the 2-week taper begins tomorrow, ending with the Grandfather Mountain Marathon. Hopefully, these 2 weeks will allow me to struggle to get to the end of Mile 26.2!