I've gone for what I suggested:

Did not start
Completed in over 24hrs
Sustained an injury
Ran out of time
Bad weather
Got as far as... (enter location in box below)

Didn't want to have too many options and there aren't that many points/tops where a misspelling would be confusing plus I can always ask. Or could have the dropped out text box actually be a drop down with all the tops and road crossings listed. Might split the "Ran out of time" as you suggest into

Ran out of time (behind schedule)
Ran out of time (got lost)

Interestingly most of those who have so far given a reason this year have finished in over 24hrs:

3 did not start
10 finished in over 24hrs
5 quit at some point along the route.

21 either haven't replied or not given a reason.
Will see how it goes.