Get on a BIKE, ROW, SKI, ELLIPTICAL or RUN/JOG.
Stay at below 70-75% of your MHR. Measure it with Watts or Kcal or Distance to MEASURE PROGRESS. Can you do more meters, kcal or watts on SAME HR? This is what PROGRESS looks like! Some call it PHYSIOLOGICAL ADAPTATION to a STIMULUS= Increased EFFICIENCY.
The feel is of an EASY workout paying attention to the HR NOT increasing above 75%.