It's a lovely race, well organised, lovely route, great local support. I've done it twice and plan to do it next year. And the entry fee is pretty reasonable compared to most of the other local HMs going on around the same time. Even I can afford it
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