I've had the best success using longer tempo runs of 7-12 miles done at a steady pace. It doesn't take much to cross the red line when doing short tempos, so I feel longer ones are better and safer. For a 16:00 5k runner, try doing runs like 8-10 miles going 6:25 for the first 4-5 miles, and gradually picking it up till the last 2 miles or so are slightly under 6:00 pace. Knowing your pace really isn't necessary though. You should start out at an easy pace that's slightly faster than normal runs. After several miles you should start to feel really good and then start to cruise. You should be running pretty fast but not pushing. It should just feel natural.