Lumbar support is for the support and comfort of the spine not for weight transfer to the hips. Most of the weight will still be on the shoulders. The weight transfer with lumbar support without a belt is minimal.
As far as a hip belt goes if the pack is light you don't need a hip belt. It just adds weight. I would say 20 lbs and less does not need a hip belt if you are the average size guy or gal without back problems. Kids should pack less weight.
If you are really worried about the weight some core exercises such as yoga or balance ball will get your torso in shape for the pack.
As always start slow and build up.
JN from Trepp?