Probably a good idea, or do it with a probe if you have them unlocked. You could also spend some more time trying to grab science from kerbin as every little bit can help.
Sandbox is great though for learning the ropes. You don't got to worry so much about dead kerbals or running out of funds/reputation/whatever.
I'll be fairly impressed though, if you can pull off getting to the mun and back without maneuver nodes
Definitely not a walk in the park given it can be tricky enough doing it with manuver nodes.