I've made a through tenon joint but the tenon is a little long. How do I plane it even with the surrounding area without the plane blade cutting anywhere other than the tenon?....if that makes any sense.
Flushing the tenon
There are a couple of ways you can approach this, but it is actually most common to take the projecting tenon down until just about flush1, and then planing right through (in the direction of the grain of the surrounding wood in case that's not obvious). As far as 'damaging' the immediate area by doing this it's not normally a problem because the piece won't yet be finished, and even if this planing is only done locally the minuscule amount of wood you remove — a few thousandths of an inch at most / ~0.1mm — isn't usually important. This is really the only way to achieve flawless results like the following, although it would be very common to do some sanding as well:
If the surrounding surface is pre-finished (and you should ask yourself why in this case) so you don't want to touch it with a plane it's just about possible to make the tenon end flush by protecting the surrounding area with tape and planing very cautiously, but it is tricky and even taking the thinnest shavings possible2 you have to be extremely careful. I wouldn't expect a perfect outcome every time doing this.
Optionally, leave it proud
Given the above one good option is not to strive to make the tenon end dead flush and instead deliberately leave it projecting. While the look this gives is not to every taste it has a long history in woodworking and is often seen as a sign of quality craftsmanship (although to be fair a perfectly flushed tenon should be too).
Usually projecting tenons are chamfered somewhat or gently rounded over, this is partly by necessity so the tenon doesn't have sharp corners that will catch on things but it's also for aesthetic reasons.
1 Often the bulk is taken off by sawing, then maybe some paring with a chisel, although these days with power sanders it wouldn't be unheard of to use sanding for the whole operation. You can also use a router to do this, see here on Startwoodworking.com, but note the use of a hand plane to complete the job.
2 0.001" / 0.02mm or under.