It's mostly trail after Richmond to Kingston then cross Kingston Bridge and it's a bit of tarmac then trail to Hampton court, don't know after that. There's plenty of options for adding a few extra miles eg loops into Richmond park, Bushy park and Home park (behind Hampton court), all trail/grass.
There's also a big crossing at the A3 at Robin Hood gate for pedestrians/bikes/horses which makes it really easy to cross over between Wimbledon common and Richmond Park, I've done the half marathons there (Wim Common) some lovely trails for running on
Don't know if you could squeeze 40 miles out of it but Wimbledon Common/Richmond Park/Thames towpath and Bushy park are not far from you and are lovely places to run, you could link them all up with a bit of careful planning and minimal tarmac
Blimey, and this thread is called Things you want to say but Can't
I loved that there were free toilets at all the gates in the park, including drinking fountains, for anyone that wanted them - even *shock* yummy mummys. It's one of the things that made it such a good place to visit. It's not particularly an issue for me personally, but could be for others. They've been free for the 20 odd years I've been visiting the park.