A few more details as promised:
Firstly, I realised a couple of hours in that given how my route ended, Canons Park was the ideal starting point, and certainly better than Wimbledon Park. We started separately as I was running late, however I still ended up starting half an hour before the others

Not sure how that happened. By the way, I
would have gone south from Wimbledon Park!
Beautiful frost in the forests beside the tracks heading up to Moor Park and back early on. This didn't take much time as I got a fast train direct from Harrow on the Hill on the way up.
One way I tried to jump a train was from Ravenscourt Park to Chiswick Park. Rather than just taking the next Ealing Broadway bound train to CP, getting out and taking a photo, then continuing on the next one, I took the first Richmond train to Turnham Green, and set out on foot to Chiswick Park, hoping to beat the first train there. Turns out there's a path through a park alongside the track; as a result I had the sad sight of the train going past when I was a couple of minutes short of the station. It meant I walked the rest, and the station guard had a go at me for taking a photo as I walked in! Then massive delays getting to Park Royal, however it's rare to have delays early on then a clear run, and it was a pleasant change!
Dumb moment of the day was when I decided it would be a good idea to run from Holland Park to Notting Hill Gate (then down to Wimbledon Park) I set off, and slowly gained on a bus ahead of me. I eventually caught it up at a stop, and feeling tired jumped on, not knowing how far I had to go. Turns out I was 50m short of the station, and the next bus stop was 400m down the road. So I had to backtrack 350m! Don't think it cost me a train though.
I couldn't find a 'normal' station sign at Hyde Park Corner. Did I miss it at a different exit? At several stations, I asked a passerby to take a photo of me under the sign, at one I was asked if the camera was a bomb before they took the photo!
I bought an extra camera from a shop in Holloway en route to Finsbury Park, and almost grabbed a sandwich while I was at it, but decided that I didn't have time in the end. The last couple of hours were frustrating as I'd saved the difficult ones till last, but I'd done my route prep and got lucky at both Stratford and Meads Lane - a quiet road near Newbury Park, and was on course to get 8 hours as I left Elm Park. However the last train was excruciatingly slow - the others also had trouble getting back into town on the District - and I took my final photo outside Upton Park 8 hours, 8 minutes and 36 seconds after taking a similar photo outside Canon's Park.
I'd been texting the others to keep in touch for most of the day, however we'd not been in contact for the last couple of hours as we raced to our finishes. I broke 'radio silence' at Marylebone, heading up to Amersham for the curry meetup, expecting to be well beaten as they had told me they were finishing at Moor Park. I was amazed to get a text back quickly saying that I would be first to Amersham. When I arrived in Amersham, they then texted back to say that they were leaving Eastcote
for Amersham. I guessed they must have finished early and gone back to Starkey's. How wrong I was.......
Soup dragon kept me entertained until the others arrived, then we discussed routes and ideas over food, before I got a lift back to Rickmansworth on the floor of Soup Dragon's car, only once having to duck when a police car passed. And work this morning!
Thanks to all the others for organising the event, having the idea, and for making my time look good
It's not a piece of cake, but one of the most interesting challenges I've attempted. Think it could be done in about 7 hours on a great day with a better thought-out route. Guess I'm the record holder for now though.
