London Marathon

depends which start pen you are put in based on your estimated finish time and  denoted by a little round circle on your race number when you get it. I would think you would be in at least pen 6, probably further back. I started in pen 6 in my first race and crossed the line seventeen minutes after the gun from the red start.

It doesn't matter how long it takes, as it's timed by using a chip on your shoe. The clock for your time will only start when you cross the line.

I started on the red start on my first one and I was so far back (pen 9) that I couldn't even hear the gun

I thought you meant "from where you're staying"!!!

So I'll pretend you did

What I WOULD say is that the DLR journey and walk to the start area is actually considerably longer than I had anticipated and you need to leave PLENTY of time for the journey if it involves DLR travel. There is quite a hill in Greenwich park too before you've even crossed the startline. Each startline is a fair old distance from the other ones and once you're committed to yours, they won't let you back through to the other ones so don't go through until you're ready.