Walk from Bushey station to Rickmansworth through Oxhay Park and then along the Ebury Way to Rickmansworth. This morning section is c5 miles. The Ebury Way is an old railway track so it's a hardcore fairly level surface. Trainers will be fine for this section of the walk.
Lunch stop is in Rickmansworth. For those with their own packed lunch there are benches outside the parish church. Otherwise I intend to go to the local Weatherspoons - The Pennsylvanian. You cannot book tables at Weatherspoons, but I've never had a problem getting a seat here. There are other pubs nearby if you don't fancy the 'Spoons.
You can get the Met Line back into central London from Rickmansworth if you want to drop out here.
After lunch I intend to do a circuit of the lakes of Rickmansworth aquadrome before heading off to the station. This will add about 4 miles to the walk. You can expect mud on some of the paths around the lakes so I would recommend boots if you're going to do all of the walk.
Get the 9:45 Lioness Overground line service from London Euston to Bushey (arrives 10:26). This is free for all those with Oyster 60 or Freedom passes. Note if you are running late you can get the faster West Midlands Train service to Bushey. However this might require you to buy an extension ticket.
Note you should exit to the back of Bushey station.