Can exercise be effective if done randomly throughout the day?

I’m not looking to get built here. I just want to increase my general upper-body strength a bit. I don’t have the motivation to actually undergo a workout regimen, particularly as I get enough activity to maintain a decent level of fitness from juggling, playing badminton, tennis, etc. What I can do is a few sets of push-ups or pull-ups, maybe some sit-ups (anyone know any exercises that don’t end in -ups? This is getting rather monotonous) as a bit of a break from homework or whatever it is I am doing. At the end of the day I will have done perhaps 100–150 push-ups in sets of 15–20, 50 pull-ups in sets of 10, etc. Is this an effective way to gain some strength, or am I getting sore for nothing? Do I have to do my exercise all at once, with a warm up and all the frills, to get a reasonable benefit?