Used to bike really far, but can't do it anymore. How much do lungs weaken with age?

Not just lungs. Everything weakens to some degree with age. Lean body mass (muscle) requires more exercise to maintain it as age advances. The same is true for cardiovascular fitness. A break in your workout routine always sets you back, but the time to recover a given degree of fitness is longer as you get older. It can be done, but you just have to be patient with your body.