One rule of thumb for determining how long your "long run" should be is to take the distance of the race you are training for and add 3-4 miles to it. The rational for this is you need to warm up a couple of mile for a race, run the race, then run a couple of miles to cool down. You also need to account for the fact that your race effort is harder than your training effort. So if you are training for a 5k you would need your long run to be at least 7 miles (3 for the race plus the 4 miles of warmup and cool down). If training for a half marathon it is ideal that you could do a long run of 17 miles. Typically your long run will account for 20-30% of your total weekly mileage.
Don't go out in week one and attempt the full distance of your ideal long run. Start with what you are able to do, then add 5-10 minutes every couple of weeks until you achieve your goal distance for your long run. Once you are able to cover the distance you can work on improving your time/pace for the long run.
Long runs should start off at an easy effort, what most would call a conversational pace. Running with someone else is helpful, but don't get sucked into running too hard if your partner is faster than you. When I run in groups we tend to run the pace of the slowest runner, at least through the early part of the run. Late in the run it is okay to open it up a little bit and pick up the pace.
So make it a habit of running a long run every 1-2 weeks!