Hey Clay,
I felt very compelled to reply to this, as this SAME thing has happened to me this spring.
Ok, so about 8 years back I was training hard one spring and POP, left knee pain. I was "loosely" diagnosised by a knee specialist with Patella-Femoral Syndrome - no scope, just from the looks/sympoms.PTS basically means "pain in the inner kneecap area" from improper tracking of the tendon..... As directed, I took a break for 6 weeks, inner quad strengthing exercises, got back on the bike and trained, but never did I ride as hard as before.
Riding kinda went out the window for the next 7 years...i'd ride hard every now and again, but never over about 15 miles or so....
Fast forward to late April this year. The bug bites me, I get a new bike - I take my daughter out with her hitchhiker bike (add 60lbs for her and that) and we hit easy flat stuff for 25+ miles. I was riding my old bike (which was too small) with OLD gear (shoes/pedals). My knee SCREAMED at me! So bad that I could barely walk for a few days. I had no idea what to do, so I basically looked into EVERYTHING from Bike fitment to shoe/cleat fitment to Yoga and Supplements.
I guess what I'm getting at is that there are MANY things that could be causing your knee pain. Seat height is a good start - Cleat adjustment? I found mine were WAY off. Bike size? I was riding a bike a size too small for me. Even maxxed out w/ a longer stem and seatpost, it kinda fit....but it just wasnt't CORRECT.
This is the only way to start - REST. It sucks. I had JUST got my Fisher and I had to take the pedals off so I wouldn't be tempted. REST!
ICE - 15-20 minutes 2x a day if it hurts. And don't forget this later, cause as you ease back in, it's a good idea to ICE after you ride longer sessions.
Brace? Maybe...i tried 3 - they didn't work for me, but the one I kept sure hold the ICE pack on great when driving home from the trail!
S t r e t c h i n g - I cannot recommend this enough. SERIOUSLY. I am so much more flexable now, and it's helped work through the pain. Do this 2x, 10-15 minutes a day. The first week sucks. The second week is even worse because your so sore. The third week is no better, but you can really start to see results. The 4th week you'll find yourself wanting to slack, but keep at it. Stretch your quads, IT Band, hamstring, groin - all of it! don't skimp - do it religiously.
Ibuprofen. It's your friend for swelling. The swelling is what's causing your pain. I recommend using it at night or when your NOT riding. The last thing you want to do is deaden the pain then ride throught it to another injury.
Glucosimine. I'm using it, seems to maybe have helped. I feel like an old person taking it but what the heck if it helps repair tissue. Flax Seed Oil. also good. The jury is going to be in when I kick these supplements and see if the pain stays. That may or may not ever happen!
Fitment - VERY important 'cos when you get back to it, you don't want to re-do it! How much float do your pedals have? are your knees at a weird angle? There is UBER amounts of info on the net. It will make your head spin. I ended up changing to a clipless system with more float and new shoes. This seems to have helped my knees.
Cadence - can't believe it took me so long to remember that! - SPIN! - low gear load, high pedal RPMs. Learn to spin, your going to need to take load off that knee.
After 6 weeks, I eased on in...and I'm still easing in...the most I've done was 3 days in a row, 6 miles, 12 miles then 14 miles - it didn't kill me, my knee kinda complained, but the pain went away after a day or so. Bring the mileage up slow. Don't expect to do what you did before RIGHT away - you can get there, but ease it in. Knees gotta last you a while!
There is no easy answer, 'cos I looked high and low for it. You have to look at the contributing factors and change the way you did things before.
ok, end of rant - hope your knee gets better - let me know what works for YOU
~åaron