Healing is one of the main greek disadvantages so the only way to heal like you said is Athena's god power or Apollo's temple upgrade.
I remember using Apollo's temple upgrade to heal only once in the campaign (I usualy chose Dionys.) and it healed FAST. In a couple seconds, I had my heroes and units completely healed. The radius however is very small, you have to be touching it. It also seemed I had to stand INFRONT of the temple entrance to heal but maybe I just didn't have them close enough? I believe it can heal multiple units however you cannot healing while fighting (I had oddyseus right next to the temple while being attacked by some Hippikons and it didn't heal. It might be possible to use multiple temples and heal, I'll try it tomarrow.
[This message has been edited by Celois (edited 10-23-2002 @ 00:50 AM).]