How to build muscle fast without weights?
If we start by running, our muscles are going to scream at us after the first session and then we will probably quit. Start slow at shorter distances, building up not only your leg muscles but also your cardio vascular system. As they both get stronger increase your pace and distance. Running, jogging, and/or walking can become something that is physically and mentally therapeutic as well as an activity you love.
Yes we all remember these from freshman year. We hated them then and probably still hate them today, but they are an effective and efficient way to build muscles in our upper body.
As discussed with running, start with smaller numbers and work your way into higher numbers as you gain strength. You should not start with one hundred as a single repetition, again your muscles will scream at you and you will probably quit.
What if you have a bad back or other injury that prevents you from doing the old army style of push up? You can use a modified form of push-ups by leaning on the wall, on the kitchen counter, or wherever to do the push-up at an angle. As you grow stronger you may be able to work your way to using the floor as your starting point.
See the comments about freshman year and duplicate here.
Crunches again are something you can easily do and benefit from. The crunch, not the old sit-up, is much more effective in strengthening the abdominal muscles and your overall core muscle group. Start with a few and add some more everyday. You can even do these in the bed, but they are more effective when completed on a firmer surface.