There are three critical physiological metrics for endurance athletes in measuring improvements in fitness; aerobic capacity, anaerobic threshold, and efficiency. Performance increases come from improving one or more of these.
Aerobic capacity: The maximum amount of O2 in ml an athlete can use in one minute/kg of body weight.
The medical dictionary
Anaerobic threshold: The level above which pyruvate—an intermediate product of anaerobic metabolism—is produced faster than it can be used aerobically. Unused pyruvate splits into lactate (lactic acid) and hydrogen ions; continued exercise above the anaerobic threshold results in accumulation of these ions (acidosis), causing exhaustion and intramuscular pain.
The medical dictionary
Efficiency: A measure of effectiveness; specifically, the useful work output divided by the energy input.
The medical dictionary
Training can improve aerobic capacity by as much as 20% through structured training over time—lots of volume at a low intensity and spending time at your current V02 Max intensity through intervals. Aerobic capacity is measured as a factor of body weight, so losing excess fat to reduce weight is a quick way to improve this factor of fitness.
The anaerobic threshold, measured as a percentage of your aerobic capacity, can be improved significantly but is sport-specific. Most endurance athletes focus their training on this element. Your anaerobic threshold can be raised to a higher percentage of your aerobic capacity through increasingly higher volume training and frequent long intervals at or around your current threshold.
Efficiency is less well understood by current science, but improvements to this metric can produce significant performance gains in longer events. Many elements of efficiency are genetic such as the percentage of slow-twitch muscle fibres and body proportions. Your technique, body weight, mobility, and strength affect efficiency. Training with frequent and intensity in your sport, strength training, and technique drills will improve efficiency.
SamuelJTanner 