He's 10 months old. I might have overexagerrated a bit, I'm not exactly sure how long it takes for him to get tired, but for example earlier, we've only done maybe 4 dumbbell retrieves and some heel/drivework before he slowed down and started 'screwing up', so to speak. When it's raining we could easily go 20 minutes doing various things and by the end his recall would be lightning fast. I take him for a few miles walk each day after some obedience and then he spends 8-10 hours in the crate (includes nighttime), most of his time is spent in the house. He's of the proper weight, with the outer ribs showing, and is on a raw food diet.
Are there any other exercises we can do to help increase his stamina? I know for sure he's not in 'peak condition', but have no idea what else to do without overtiring him.