Hi there,
I have to agree entirely with Ron. If you use three 9v batteries in series to get 27v (good idea BTW) and you are getting 1 hour run time, then using one of the SAME 9v batteries and a DC to DC boost converter then you will only get 20 minutes run time from that single battery. That's because when you boost the voltage you loose ampere hour capacity as Ron already stated. That's life, but there are other things you can do.
For one, you can start with a 12v lead acid battery that can have many times the ampere hour (run time) capacity than a little 9v battery. With that you can boost up to 27v or whatever you need. You'll still loose some ampere hour capacity, but the loss will be a little less and besides the lead acid battery will have MUCH more capacity to begin with. For example, if you take even a 1.5 ampere hour rated 12v lead acid battery and boost it up to 27 or so volts you'll still get at least 0.5 ampere hour from it. An additional plus factor is that you can recharge the 12v battery so you can use it again.
Alternately, you can string rechargeable batteries in series, as many as you need up to something like 20. If you use 1.4v rechargeable cells that will give you 28 volts.
If you use 9v rechargeable's (not as good though) you can get away with three or four in series and then recharge as needed. The rating on these batteries isnt that good however, whereas with 2000mAh NiMH cells you can get four times the run time you are getting now with the three 9v batteries. Nice right?
If you do choose 12v lead acid (sealed gel) you have to get a charger with it so you can recharge it correctly. You also dont get as many recharge cycles as you do with NiMH cells.