The central goal of "Hard Time" by MDickie is to survive a prison sentence. The player's four core statistics must be kept up to some degree, but the Strength and Reputation stats are most important for survival.
Strength is important to survive fights, and Reputation is important to head them off. Agility and Intellect also help somewhat, but are not as vital.
Reputation can be gained quickly by picking fights with guards or even the warden, but there is a trade-off of possibly getting a sentence increase or getting injured. Strength can be boosted by working out. Maintaining strength is also helped greatly by getting a full night of sleep, taking naps during the day and getting regular meals without eating too much food and throwing up.
It's also wise to avoid aggravating gang members. This can lead to a hit being put out on the player. Since guns and swords are somehow readily available in the prisons of "Hard Time," this can easily turn into a fatal mistake. If the player is forced into a fight and can't find a weapon, they should use their "super attack" to even the odds.
Sub-missions aren't entirely necessary to survive a sentence, but they can help by providing big boosts to stats that are falling behind.