Well done on London Yvonne. It does sound like my idea of hell tbh....I have a GFA time for London but nothing about it appeals. My husband is like you and done both and he is also firmly in the Brighton camp.
Hi Khanivore, I've only been running just over 4 years and I had just done 3 road marathons before my first ultra so not that much more experience than you.
I'd recommend trying an off-road marathon before your ultra if you can fit it in (not sure where you're based but I did Beachy Head marathon and Steyning Stinger) as they'll really help you with 'time on feet' and also learning what works for you nutrition wise.
It's definitely possible to do both road marathons and ultras but you'll probably end up veering towards one, I'm much more in the ultra camp these days but am sure I've got a few road marathons left in me!
I've heard very good reports about the Thames Trot, I guess there's very little navigation involved - I totally under-estimated navigation on my first ultra and ended up running 57 miles instead of 51! My next ultra is the NDW50 next month which apparently is well sign-posted so hopefully I'll only run 50 miles this time!