That's really a judgment call on your part. Do you think you can run 10K? Do you think you'd have to work so hard that the 10K wouldn't be fun? Are you willing to do walk/run intervals if you're not in shape to run the full distance? Would you feel like a slacker if you dropped to the 5K?
I've dropped to a 5K twice, both times from half marathons that I wasn't ready for. But 10K isn't a half marathon, and if you regularly run 5K you might be able to run 10K without a problem. You know your own body better than we on the message boards do.
"Discipline is remembering what you want. " - David Campbell
Max Lifetime Weight, 221
SP Start Weight, 196.6
Initial SP Goal, 175
Current Goal on Ticker, 162
| current weight: -0.4 under