No there is nothing wrong with running for time. Just get to where you can run for however long your goal time is then figure out (eventually) how far you are covering during that time. With the goal being to make that distance longer within the same time. That is all long term depending on your current fitness level. I know if I am going on a run I like to be running at least 30 minutes. An hour is a little wearing, and I am not saying I am fast no not at all. My last fitness test I ran three miles in 25 minutes, which is kinda bad. I need to get back into the 22 something range. I would be happy with that. When I was 23 I once ran a 19:01 three mile but I was in pretty good shape then.