Aerobic exercise capacity is strongly linked to 02 max which is the product of the cardiac output ( stroke volume X heart rate) multiplied by the A-v 02 difference.
Five middle aged men who underwent aerobic training at age 20 in 1966 participated in a 6 month endurance training program in 1996.Their average 02 max increased 14% but there was no change in their maximal cardiac outputs. Their A-V 02 difference did increase by 10% accounting for the entire improvement in their aerobic power.
The increase in mitochondrial function brought about by aerobic exercise is well documented and the more mitochondrial the higher the 02 uptake by the exercising muscles.
1)McGuire, DK et al A 30 year followup of the Dallas Bedrest and training Study11Effect of age on cardiovascular adaptation to exercise training. Circulation2001 Sept 18 104 (12) 1358-66