Cardiovascular activity. Start by doing an aerobic activity, like walking or running, for a sustained 20-30 minutes, four to five times a week, says Bryant. To ensure you're working at an optimum level, try the "talk test": Make sure you can carry on a basic level of conversation without being too winded. But if you can easily sing a song, you're not working hard enough.
Evidence from HIIT studies conducted under controlled laboratory conditions has provided proof-of-concept of efficacy [9]. However, it has been argued that HIIT has high efficacy but low effectiveness [16], and long-term exercise interventions carried out under free-living conditions have been asked for to investigate whether HIIT is feasible as a public health initiative among older adults [9, 16]. Our data showed that both training groups reported on average more than two exercise sessions per week throughout the year. Approximately 60% of the sessions in the HIIT group were performed with a self-reported high-intensity (≥15 Borg scale), indicating that older adults are able to perform HIIT over a long time-period without strict supervision. However, women had a lower proportion of sessions with high-intensity exercise compared to men. This result is in line with previous findings that women (aged 60–67 years) are less likely than men to prefer vigorous physical activity [23].
Mixing up your workout routine from time to time is very important for avoiding the dreaded plateau which is basically your body’s way of saying “I’m bored!” and it’s a big issue with a lot of people who have been on the same workout routine or fitness program for a while and really aren’t seeing the results they want. Your body needs stimulus from a variety of sources which includes everything from different reps and sets schemes to various training styles. If you’re struggling with building muscle mass or you’re having a difficult time losing body fat, then your primary goal should be to mix it up a little and start adding variety into your routines. By doing this you can truly shock your body into change since it will be receiving new stimuli from different sources. Use our extensive exercise guides on this page as a roadmap to help you reach your fitness and physique goals!
When intensity is high, it is physiologically impossible to work out for a long time. Doing more exercise than is minimally necessary to stimulate adaptive changes (or to maintain a proper level of fitness) drains bodily resources and compromises recovery. A properly performed workout should take no longer than 45 minutes, which if done in a gym can also include some time spent on a treadmill at the end of the workout.

Aerobic exercise induces short- and long-term effects on mood and emotional states by promoting positive affect, inhibiting negative affect, and decreasing the biological response to acute psychological stress.[45] Over the short-term, aerobic exercise functions as both an antidepressant and euphoriant,[46][47][48][49] whereas consistent exercise produces general improvements in mood and self-esteem.[50][51]

The Alfredson protocol for Achilles' tendinopathy is actually two separate exercises.  To perform the exercises, you must have a small step or curb on which to stand.  Be sure to check in with your doctor or physical therapist to ensure that it is safe for you to exercise and that you are performing the exercises correctly.   Here is how you perform the Alfredson protocol:
Exclusion criteria included major diseases or conditions such as severe heart disease, uncontrolled hypertension, obesity, osteoarticular pathology, and neurological disease. Criteria were evaluated on the basis of clinical history, resting ECG, and physical examination. Participants maintained their lifestyles and were instructed not to take part in any other physical programs throughout the study. At the time of the initial design, the study consisted of a 12-week randomized controlled trial with a frequency of 3 times a week, 36 sessions in all, ending with a new assessment of their wellness and the potential persistence of the results on functional/physical capacities.