A four month tour with no prep should be no problem unles you're way out of shape. A more likely problem would be mistakes made due to lack of riding time. For example, if you're not used to riding in trafic (I've done all my tours in Japan where trafic can be a problem in cities) and you encounter lots of trafic you might not know how to deal with it. Also, if you're not used to riding a loaded bike, this could be a problem. It's much more important to prepare by spending a couple hours a week riding a loaded bike than twice as many hours getting your body in shape.
By making costly mistakes that will likely result in knee surgery I've learned the following:
First, I must say that distance is really a poor judge of what your body has been through in a day's ride. There are many more important factors such as wind, the ammount of weight you are carrying, terain, and weather. even what goes in and what comes out (or doesn't come out) of your body has a bigger effect on your ability to continue a tour than the distance that you pedaled the day before. I've had days that i felt terrible after a previous day of 60km, and I've had other days that i felt great after a previous day of 200km.
day 1: you'll feel great to begin with, but don't let the exitement get to you. I hate planned tours, but it's a good idea to plan an end point for the first day and stick with it.
day 2: your butt will hurt a bit unless you have a really great saddle and suspension system (i didn't). your legs and upper hips and lower back will be a bit tight, but you'll feel great again after the first 45 minutes. it'll feel like a ride in the park, but you'll wanna actually decrease the milage a bit.
day 3:your butt will hurt even if you have a good saddle. especially if you haven't ridden much before you started the tour. if your shoulders/upper back start hurting a lot you need to adjust your riding position a bit to make your arms less extended and elbows bent more. sheldonbrown.com has some good articles about riding positions.
day 4 and 5: these days will suck no matter what you do. don't drink too much caffine on these days or you'll think you're better than you really are. always do light stretching. light. and if your hands start to go numb make sure you concentrate on keeping your elbows bent when you ride. don't ride with locked elbows. stop a lot and take lots of pictures and enjoy the scenery ^_^. if you're a determined person make sure to turn off that part of your brain. take it easy.
day 6,7,8: if your knees are going to cause you probablems this is when it will happen. keep your stroke straight. don't move your knee closer to or further from the center of the bike when you pedal. if your knee swings a lot when you pedal, the inside or outside tendons will start to hurt. also, as a rule of thumb, if the front of your knee hurts raise your saddle a little. if the back or your knee hurts lower the saddle. there are too many possible reasons for knee problems for me to even think about. these are just general ideas. a little knee pain isn't terrible, and it will probably go away by day 10 or so. but it's best to call it quits for the day if you feel constant knee pain, even if you've only pedaled a few km. if you let your knee rest it will usually feel good the next day, but if you push it too hard it could take months to heal, or maybe it won't heal on its own (like mine). I've taken a few friends on tours and most of them feel slight to considerable knee pain around this part of the trip. just take it really easy. oh, and never ice your knee if you plan on riding the next day. keep it a little warm.
day10 you'll probably feel pretty good around day 10. you might go through a high and a couple days of low before you finally hit your grove.
since i've already written much more than i planned on, i might as well include my top 10 list. or maybe i'll call it the biker's worst enemy list.
1. wind. (i rode by the ocean a lot). it stops your momentum. momentum is everything on a heavy bike.
2.tunnels
3. tunnels with trucks
4. narrow tunnels with trucks coming from both directions
5. steep, twisting decents in the rain with loaded bike and cheap brakes (this caused our worst accident. i was touring with my brother).
6. wheel berring problems due to lots of rain riding. if you spend money on anything spend it on decent hubs and brakes.
7. wet socks (if you are putting on the socks wet this jumps to #1 ^_^).
8. walking into an air-conditioned grocery store when you are soaking wet.
the best:
1. meeting lots of nice people especially when you depend on them to be able to continue the tour.
2. wind. if it's behind you.
3. freedom
4. the best sleep ever. even in a tent every night.
5. no tv. unless....
6. great time with touring partner.
7. any food tastes very good.
8. you learn lots about yourself and other stuff and other people. i ended up a changed person.
i guess i'm assuming that you are novices at touring even though you may not be.
if you're not sure whether or not to bring something, don't bring it. and if your bike feels too heavy after three days, it is too heavy, so start throwing stuff away. of course, if you were touring in a very rural area i might not suggest this.
take it easy.
david