Manchester is a great marathon. Did it last year and have done London twice. Prefer Manchester. It's a real "runners" race rather than a giant fun run. What the crowds lack in numbers they make up for in noise. You'll love it.
I am flying out on Friday from Stansted and coming back the day after the race so not too much Mythos for me afterwards . Have booked a hotel near Syntagma Square so near the coach pick up point and not too far to stagger at the end ! Luckily friends with an ex-Athenian but she did comment that it will be really warm at that time of year !