Randy, I believe you can do tempo runs some during a base building phase. I’ve certainly done it. Be careful to take it easy on the intensity while you are increasing mileage though. You probably know this, but increasing intensity AND mileage can get tiring.
In my opinion the best training you can do for a 5K is V02Max interval training. I believe intervals of about 1000-1200 meters are ideal. I usually do about 5-6 per session. V02 max pace is almost the same as 5K race pace. I would not recommend doing this while your building mileage. You CAN do it, but you will probably have some days of tired legs.
I’m sure there are others here that can chime in, but I guess you have to ask yourself how important a good 5K is to you.