a lot more does not automatically translate to a lot better. You need to start practicing BETTER and as you build better habits, the endurance goes up.
Practice in my book consists of several things - none of which are marches or chop busters.
We start with a daily routine - long tones, slurs, scales and easy tunes. Then comes a block of repertory played musically - not played to annoy or destroy. The last block is technical studies like from the Arban or Clarke books. You can double your playing time by simply playing softly.
Every brainlessly played note makes you WORSE because it trains not caring and bad body/brain use. EVERY NOTE COUNTS.