Tips for increasing length of tempo runs

Run 7-8 miles instead of 4-5 miles.

K.I.S.S. applies here.

Just run a little slower. Honestly, that's about all there is to it. I can't really give you an exact pace, but somewhere in the neighborhood of 5:50-6:00 should be fine, but don't worry too much about it. The point is to get in a strong, controlled effort to improve your endurance. Long tempos can really help some people, so it's good that you are giving them a try. However, they may take a little more out of you, so I wouldn't do them with the same frequency that you did they 4-5 mile tempos. Don't be afraid to mix it up.

There's really not much more to be said on the topic. Run a bit slower and then run further.

If you normally do 4-5 miles in 5:35-45, just start out around 6:00 and try to work down to 5:40 or so.

I should also add that with such a tight range of paces, I have a strong suspicion that you're forcing the pace a bit, ie. going too fast when you're not feeling great, and not going fast enough if you're just "on." Tempo, broadly defined, is going to be by feel. Look up "Summer of Malmo" and read what he says about "running to the barn." That may be valuable to you.