...Now Continued:
DO NOT ATTEMPT THIS TRIP WITHOUT A GOOD MAP
www.mapmyway.com is the best one - It's a German printing and laminated and can be found in finer bookstores and finer street vendors in most towns in Thailand. Buy one in Pattaya and study it so as to follow my suggested itinerary and to modify as you see fit.
Suggested Overnights:
250 kms is a good days ride in Thailand. It's dark by 6pm you want to avoid dark riding unless you're a night rider. Rural roads are not lit like in the UK though the towns are. So budget your daily riding to a max of 250 kms.
Overnight in:
1) Ayutthaya (and you'll need it from riding from Pattaya)
This route to Ayutthaya has avoided all BKK traffic buzz.
2) Nakhon Sawan UNLESS you are hard rumped and can get to Sukhothai
from Ayutthaya, but I'd take it easier and stay in N.Sawan.
3) Sukhothai Town (or Phitsanulok as mentioned above in prev post)
4) You can make it to Chiang Mai from SUKHOTHAI - a full days ride.
-When You Come to a Fork in the Road, Take It.