A bunion is "an abnormal enlargement of the joint (the first metatarsophalangeal joint, or MTPJ) at the base of the great or big toe (hallux)." It's usually either an inflammation of the bursa around the joint, or bony ridged growth around the ends of the bones meeting at the joint. A dorsal bunion, which forms on the top of the foot, limits upward bending of the big toe. In the running stride, this limits the push-off from the big toe unless the foot or leg does something to overcome it.
There's a saying I remember from structural analysis - stiffness attracts load. If you push on an otherwise flexible wood-frame building that contains one steel column, the steel will take up most of the stress because it cannot be displaced as far as the wood without requiring more force. If all the toes are flexible but the big toe is stiff, the big toe will do a disproportionate amount of work.
The muscle and tendon which plantarflex the hallux come around the inside ankle bone (medial malleolus) like a pulley and attach about three inches up on the posterior medial side of the tibia. This insertion point will get inflamed and painful from all the increased work the big toe has to do, and that's the case with my left foot.
The only way to correct the dorsal bunion is with surgery called an osteotomy, of which there are several types depending on the technique used to cut and shorten the first metatarsal, and named for the surgeons who invented them. There are ways to reduce stress on the hallux, one of which is to simply raise it with padding. This allows the big toe to remain straighter on push-off and moves the push-off point from the toe to the front of the metatarsal. I've had this type of padding before but forgot that it may be of some use.
Time to call the podiatrist and see if he can fix me ASAP.