Well done from me too! But, like Alarm, I would say "Respect the pain. It hurts for a reason".
From doing personal challenges, running and swimming events, in my younger days, I moved on to more leisurely but, perhaps more challenging events.
It happened by accident, rather than a thought-out strategy.
Just talking about the "Three Peaks" to a friend in the pub one evening, it developed into a regular annual challenge.
We'd 'spread the word about' our impending challenge to welcome any newcomers or charity fund raisers to join us. We had a hardcore of about six who would do most challenges, but occasionally we'd have up to fifiteen join us.
We started with the Three Peaks for a few years, then we'd choose different challenges such as the Fourteen Peaks, Coast to Coast cycle, Alpsitz, Zugspitz (which invariably coinicided with Oktoberfest). My final participation was Mont Blanc but the group went on to bag a few peaks of the Cuillin Ridge. Scotland is not my favorite place during the midge season!
After each annual event we'd have a de-brief, discuss and select the next years event. We did all our own organisation. We considered the advertised challenges to be more of a business than a charity fund raising event.
We'd organise some days training, with the stipulation that all participants must attend at least one or two, to avoid any embarrasments.
We laid down some ground rules for fund-raisers, e.g. verifiable accounts.
We also welcomed non-participants to help out with support, etc.
As most of the hardcore group were employed by the same organisation we were able to leverage some sponsorship. This varied from transport costs to out-of-date consumables.