The dash temp gauge reads in the middle between hot and cold within a few minutes of running. However, it takes about 20 minutes for hot air to come out the front heater vents when the heater is set to maximum heat and the fan is on.
I noticed that the coolant was low so I filled the coolant using the reservoir up to the "full" line. I tried running the van again and took it back up to normal operating temperature. No change (still no heat). I then located the heater core inlet/outlet tubes. The easiest access to them as far as I can tell is from inside the van, they come in to the right of the gas pedal. I touched them, one was very hot, the other was quite hot but I could hold on to it without getting my fingers burnt.
Given this, what should my next move be? Take it to the mechanic? :)
Update: I added even more coolant from the radiator (after waiting until it was cold). It took quite a bit. It has been pointed out to me that I have a coolant leak that I need to get checked out. After adding the coolant I again ran the van, sitting in the driveway and revving the engine to about 3000 rpm for a few minutes. The temp gauge read normal (midway between C and H) and the heater core pipes were both very hot to the touch. I turned on the blower and initially it came out hot but then went back to being cool again after a short while. That was last evening. However, this morning when I took the van to work, the heater blew hot air quickly, as it used to. At this point I'm not sure if I'm dealing with an intermittent problem, or if it's working properly now.
Another update: heater is still working fine. Had to add yet more coolant though as reservoir still showed significantly below "full" level. This morning the check engine light came on. Taking the van to the mechanic's to see what's going on (might be coincidence, don't know).