You really won't be last, I've seen loads of rfl's, and there are always people who are struggling to walk the distance and finish well over an hour. The atmosphere is good and a large proportion of the runners wil be first timers.
Daughter gymnastics, son football, they're in good hands, get out for a run. When our kids were between 8 and 15 or so I used the time they were at football to get a good run in. You might find that some of the other parents want to join you, we had our own little group in the end.
I don't have an answer, just sympathy. I have a 620, and feel the same way really. I used to like plotting out a trail route and downloading it to the watch and setting off into the countryside. I could probably do it now but the whole interface is so cumbersome I can't be bothered.