Odd - 18 miles that didn't hurt too much

Yesterday I set out with no small amount of nerves for my longest run of the marathon training plan that I have (for the most part) been following. It called for 3hrs, with 2hr 15min at long run pace and 45mins at regular pace - which I adapted because of routes, etc. to roughly 20km long run (just below marathon pace) and the rest at whatever pace I would feel right very slightly faster than marathon pace. Trying to block the run out into two clear sections.

In the end I managed most of my plan, with the first 10km feeling weird, too easy and very nervous at around 6:29min/km. The second 10km loop (I ran around Cardiff Bay twice in full loops) I began to realise was on a good schedule, I had reserves, and once I had bought a bottle of water (my hidden one was discovered and taken, whoops) things were looking on track at around 6:28min/km.

I'd noticed that I had the itch to run faster but was relieved to look back at the GPS track afterwards and find I didn't succumb in that second loop. The final section the run home started out - well how can I put this? - too hard! I turned for home thought about the pace change and seemed to try and do that in the first kilometer or two, which left the rest of the run (just over 6km) feeling that bit tougher than it should have. I managed something like 6:10min/km for the third block.

Today I feel great, no niggles, no aches (out of the ordinary), and just the appropriate amount of tiredness. I am left a little worried about how an inadvertent pace burst made the last 2km harder than it might have been, and need to make sure that if I try any pace change at all in the race it is far better measured out. It brought home to me that one kilometre kicked out too hard can have serious repercussions down the line. Overall though I am finding it odd that it didn't hurt as much as I'd feared... maybe there is something to this training plan thing?