You'll be fine. Just take it easy and break it down into chunks, don't think of the whole distance or it'll seem very daunting.
At the start just think about getting to Sudbury, from there just think about Dunwich. Same for the way back.
Sage advice.
Although my current plan is to not actually stop in Dunwich, if we ride into dunwich then turn straight back around and get 20-30k back towards London before we stop I think it'll remove one of the biggest mental challanges.