Quite simply one of the best things you'll ever do! I've been lucky enough to run London Marathon twice, both under charity Gold Bond places, and the atmosphere and crowd support is outstanding.
The race information prior to the day is very comprehensive; it's well worth going to the 'Meet the Experts' day, particularly if it's your first marathon, and the Facebook live videos by Martin Yelling are excellent for settling any training worries and pre-race nerves. You have to visit the expo to pick up your race pack and this is an event in itself, with many stalls and activities to take part in. Beware the expensive official kit though ~ not a place for bargains to be found!
On the day of the marathon the marshals are excellent from the moment you leave the station to when you arrive at your appointed start area. There are hundreds of portaloos provided, but do go straight to one as you arrive and bank on needing to go again before you actually set off.
Once you are in your starting pen the atmosphere is electric and the route itself takes in some of the most recognisable sights in the capital (as well as some not so memorable!)
Once you've finished the process for medal, race goodies and bag collection is seamless. There is a letters system for pre-arranging to meet you friends and family, but be prepared for moving around the crowds to take five times as long as you think it will. You'll get to cross a road eventually, but it's all part of the fun!
Bucket-list stuff and always a privilege.