I use a few rough guidelines to keep myself on track:
1. Depending on your body and fitness, it is roughly 1 mile of walking to equal 3-5 miles of biking. So if you can walk 1 mile, you should be able to bike between 3 and 5 miles. Someone like Kent Peterson is definitely in the 5 range. Most of the time, I'm closer to 3.
Right now, I can walk about 5 miles in a day, tho my knee is not pleased with it. So somewhere in the 15-25 mile range is not unreasonable for me in a day.
2. Your body's fitness is a bit like a bank. You can't force your body to do more effort than it is capable of. So if you regularly bike 4 miles each day to work, making a big increase in effort is likely to lead to a very pissed off and upset body.
3. Your body thankfully is not a vindictive sonofagun... If you normally bike 4 miles to work each day, and take the week off to do a 20 mile ride on the weekend, it'll think that is pretty ok. You asked for the effort all at once instead of a bit at a time, and most of the time, your body will be ok with that. Again, bodies vary on this, and it might be you can do a smidge more or a bit less.
A "normal" week's effort for me is 1-2 trips to the grocery store (about 4 miles), a trip to the library or two (about 4 miles), and a trip out to Anna's for knitting (about 20 miles). So that means if I'm in good shape, I can pretty easily do 38-40 miles in a week, and I'll get all the basics done. It looks like I'm on track to start the year off at "normal". I won't know for sure for a week or two, since I don't count myself as ok until I've proven I can do something consistently. Bodies are huge fans of routine.
4. If you can do 20 miles in a week, a few weeks of pushing it to do 22 (10% more) is a big deal to your body. Every time you push your body for more, it has to rebuild itself. It can only work so fast on the redesign. It is really important to be fair with your body, and assess yourself slowly.
Right now, I'm on my second week of assessing myself. Yep, short errands are fine. Yep, long rides (done with care) are fine. Yep, looks like 35-40 mile weeks are within reach. But I won't be done assessing "where I am now" for another week or two. Then comes the fun part... pushing for more. If I come out with a verdict of "40 miles is ok", then I can push myself to about 44. I get to sit at that mileage for 3-4 weeks. Then I can push again, to 48 or 49. A month later, another bump, probably to about 54. If I don't hit the goal 3 weeks out of the four, odds are my body won't think we actually made the jump. If I'm dedicated and aggressive about my goals, there's a decent chance I can be over 60 miles a week. In August. The odds of me joining the 100 miles a week club this year are pretty low, since I usually end up phasing out riding in favor of walking sometime in October or November.