I just finished my 4 week solo tour of the eastern US (well, to Denver, CO) and there wasn't a day that went by that didn't have some time where I regreted it. But let me tell you, the feeling at the end of the trip is totally worth it. To look at the map and say that my legs, those little stubby things got me all of those 2500 miles.

I remember one day in Missouri (I was making my own path the entire way, and the section of missouri I went through had the WORST roads I had gone on) everything was going wrong. I almost got hit 3 times, one resulting in me going off the edge of the road into a 3 foot ditch, the other resulting in almost slipping on a grove in the pavement at 15mph, and the last I was forced into the gutter and jolt messed up my handlebars (thankfully it was all fixable). All my close calls in the entire trip happened in a 1 hour, and let me tell you, I seriously could not think of any time in my entire life where I felt worse. I was so close to calling a cab to take me to the nearest airport. I hit the lowest low in my trip. And to add insult to injury, the traffic on the road was so high (and of course there was no shoulder to speak of) that I had to walk my bike 5 miles in the rain before I got off that road. Not only that, but the reason that I was even going to go on that road is that the offical Missouri "bike" map (it don't believe it is completed, but you can download some of it) said that there was 4' of paved shoulder on the stretch of road. But there was NOTHING. That made me mad like nothing else. This map was supposed to make cycling safer, but it probably lead me to the worst road in missouri (and no offence to people who live there) and that is saying something.

But when you hit the bottom, all you can go is up. The next day was awesome, I did 132 miles and it felt like nothing, I could have done 50 more if it wasn't getting dark.

Anyway, like I said, it is totally worth it in the end. The things you do and the people you met when cycling gets tough makes the trip (and life in general) worth it.
That said, quitting isn't the end of the world, just make sure that you really want to stop.


Edit: I just remember a funny thing that happened. As anyone who has toured solo before knows, it can get lonely on the road, sometimes you haven't had a real conversation with anyone besides yourself for a few days. I remember that one day in Kansas (which had the best roads and the nicest drivers) a car was pulling into the road and I did the little hand wave to make sure the driver saw me, and I remember thinking, "hey, human interaction."
You know you are really lonely when waving to a driver counts as interaction.