Shine half marathon

[Fab39ish]

Hi I am contemplating doing the London shine half marathon with a 7.30 start. Just worried about whether a lard arse can complete it and be on the tube by midnight.
I know it will be tight as I currently walk St about 3 miles per hour but really don't know how far old Billings gate is from monument
Any advice from people who have done it appreciated.
I will be training and my current tracker shoes I am doing about 12,000 steps per day.

Could you get an Uber if you miss the tube? I did the full marathon Shine a few years ago. Anticipate 20 additional minutes for a slow start due to the sheer volume of people crossing roads. They don't close roads so you have lots to cross and have to wait for green men etc. Fitness wise you'll be fine.

I guess that could work. Need to check overground trains too. Thank you.