I let the puppy set the pace - do as much free-style play time for exercise as you can - on soft surfaces, dirt and grass.
Start with no more than 10 minutes of exuberant play time, and work up from that - following the puppy's lead - she'll let you know when she's tired.
"Never walk a mastiff further than you can carry them home."
I imagine that may work for a mastiff+X, too.
You can practice "follow me" and do some ping-pong "come" games... for that you need a second person... unless you also teach "wait" as a first command...
Avoid repetitive exercise, like jogging - especially on hard surfaces.
Don't forget to also exercise the brain - learning new words and rules can also make for a good, tired puppy.