I can now tell you what 18 miles feels like. It mostly feels like a sort of dull ache, with a touch of sunburn, and a whole lot of sleepiness.
I set off this morning on this route - another big loop of London made up of sections of paths I've walked before. That really helps as, theoretically, I don't lose time or pace checking a map every five minutes.
It was quite warm out, but not as oppressively hot as it has been recently. I stuck to shade where I could, and set off at a good pace. It was at about mile 12 that my knees and thighs started to ache a bit and they carried on aching from there. My hands and wrists decided to swell quite heavily, which was funny but not particularly helpful, and yet somehow I've still escaped any kind of painful blister. I feel lucky.
The best moment of the day was towards the end of the long drag along Regent's Canal, which is very pretty, but even pretty views get boring after several miles. I was really tiring at this point, and wondering where Limehouse Basin had hidden itself, when I looked up and saw Canary Wharf looming over the buildings in front of me. It gave me such a grin on my face and a spring in my step that I bounced my way down that last mile to the Thames Path.
On arriving home, I sat on the sofa for long enough to eat a sandwich and drink a milkshake (thanks for the advice, Clairey) before retiring to the bedroom and promptly falling asleep for an hour and a half. It was quite possibly the best nap I've ever had.
I woke up with a slightly clicky right ankle and a nice lobster pink glow to my legs. No idea how I'm going to feel in the morning, but luckily it's a rest day.
Next walk is 4 miles on Monday. I don't think I'm going to dread that one anymore.