I really feel the practicality of advice is best summarized by jiarby, Local 357, rowuk, in their above posts. The only thing I might add is motor tone can build in function if the duration of practice is increased by about 20% per week (up until when the earliest sign of maximum effort is achieved - i.e. when fatigue sets in). So if you can practice for 20 minutes at a time over a week, increase the next week to about 25 minutes. Then the following week, step a session up to 30 minutes… until you reach that maximum limit.