If I had torn my achilles tendon I would be fearful of repeating the injury and being imobilised for months , putting myself further away from where I wanted to be
Pretending cycling is achilles friendly, or at least safer , then its got to be a mix up of intervals, hill reps , hill sprints , and some Tabbata style reps
Needn’t take very long , 20 – 30 mins Tues and Thurs with a longer 2hr effort at the weekend
Anything that gets you really blowing , and uses the same muscles as cycling will have an effect. Although improving basic fitness quickly isn’t easy unless you have unlimited time and resources