Either you are pushing yourself too much which if so you are going to damage muscles which will take a long time to heal which means you'll be off the bike a long time; OR, you're not spinning at a higher level of cadence, called spinning, which should be between 80 to 100 revolutions per minute in lower gears, if you're pushing, or mashing, in higher gears and cranking at 40 to 60 rpm's you put too much stress on your leg muscles. There is a method of training to improve power that does have 2 days out the week, spread out over a week, that all you do is mash that day, but you ride shorter distances when you do that, and you don't do that you hurt so much you can't ride or have to get off the bike to rest. The rest of the 3 to 4 days you're spinning at the higher rpm's and on those days you can ride longer distances, but again you don't ride till you are so tired you have to stop to rest.
The bike fit issue won't cause that severe of a reaction unless you have the seat so low that your knees almost hit your chest which I doubt you're doing. I'm not saying bike fit is not important, but unless the bike is severely poorly fitting to you the cause of your pain is more than likely not being caused by the fit, AGAIN, you may see some very minor improvements you get the bike fit more dialed in, you can either do that professionally at an LBS or see these web sites and try to do it yourself first before spending $200 plus parts, which is how I've always done it-by myself:
http://www.bikefit.com/s-13-road-bikes.aspx
http://www.cobbcycling.com/comfort-issues/mens-comfort-issues/ (scan down the page till you find your particular issues, but you know need their saddle to feel good!)
http://cyclingtips.com/2010/04/science-of-bike-fitting/
Muscle soreness that stays with you the following day IS NOT from lactic acid build up, lactic acid is completely washed out of your system in about 60 minutes after you stop riding. If you are sore the next day, but not in pain, then you should keep your legs moving but at a leisurely pace. Muscle pain while you ride is more than likely from over using under conditioned muscles, in other words you're pushing them to far too soon without the proper training to build them up.
If the pain only exist only while riding, then lactic acid could be part of the problem, but that is usually a burning sensation and not a pain. Read this site in regards to that:
http://www.getridofthings.com/health/athletic-health/get-rid-of-lactic-acid/ Also make sure you stay well hydrated, you should drink 12 to 16 ounces of water before you ride, then consume over time about 8 ounces every 20 minutes (personally I take in my water in smaller gulps, about 2 ounces every 5 minutes); make sure you are breathing deeply through your nose using your diaphram (which means you need to do core excercises to improve your breathing) and then exhale out the mouth. You should also try working out at least 5 to 6 days a week with 1 to 2 days off as rest days and those rest days are spread out over the week not in succession.
If you get the burn feeling while riding simply back off the intensity and spin easy till it goes away.
Also immediately after riding you should first stretch out, then consume something that is high in protein. I happen to like Met-Rx Big 100 Colossal bars, their high in protein and filling, I carry these in my saddle bag to eat along the ride and eat another one when I get home. As food goes you should fine foods that are rich in magnesium, fatty acids, and vitamin B's, get this in food and NOT a pill!
You can also use baking soda but be careful with this stuff, all you need is 0.3 grams per 2 pounds of body weight in 12 ounces of water, for example a 170 pound person would only need about 1 ounce of baking soda; do not go more than the amount suggested (depending on body weight of course), in this case more is not better. (There are some web sites that suggest using more than I suggested, as much as twice as much, I disagree, it's better to use less and not be worried about overdoing it) You must also make sure that the baking soda powder is COMPLETELY dissolved before drinking it, nor take when you are over full with food or drink. Don't forget baking soda is high in salt so if you have issues with using too much salt don't do the baking soda routine, it can cause hypertension, congestive heart failure, kidney stones, and kidney failure, therefore do not use it at all if you have a history or family history of such problems. Baking soda is a last ditch effort in my opinion, and if doesn't work then stop using it.