First, don’t run on a treadmill. It can be deceptive (it makes you think you are faster than you are). Also, it is really really boring (So much more enjoyable to drive your car to an unknown place and start running. Once you can sort of judge distances. It fun trying to find your way back to your car. Park traila and local cross country trails are nice as well) and I really don’t think you get the kind of work out you do just running out on a trail. As has been previously stated mapmyrun and Google earth are great tools to find out how far you have run and to map runs before you go. You can use this to pace your run overall. However, keeping the same pace throughout the entire run is something very difficult to do for some and easy for others. I was one of those people it took a long time to even get decent at it. If I were you I would get a watch and run on a track for at least a week or two. This way you can know what your pace is per 400 and start understanding what the pace you want feels like. However, this all depend on what you are running. If you are training for an 800 your pace will be completely different than if you were training for a 5k which is still slightly different than your pace for a longer run. So, first know what you are training for and what you want your total time to be and then break it down to find out what you should be shooting for each lap. You could either run on premeasured trail with mile marker or when you are mapping your run try to make each mile at an intersection or street. This way you can figure out your mile time for each mile, as well as, total time. Hopefully, this helps.
Other words of wisdoms, if you are just starting to run. Drink plenty of water (despite contrary belief you can’t drink too much) If you forget to drink enough water don’t chug right before you go for your run. If you do you just give yourself a bad cramp. Eat bananas or take potassium pills, drink an electrolyte beverage such as Gatorade afterwards not before your run, stretch, get good shoes. Shoes normally wear out at 400-450 mile, if you don’t get new one you will start screwing up your knees, hip, ankles, ect. Make sure you warn up and stretch before tempo runs. If you don’t you are likely to pull/hurt something.