You can become very fit exercising moderately, and catching up from where you left during the lockdown. Measured variation and determination are the keys.
Keep track of the distance you are traversing, the difficulty level of your path, and the time that it takes to complete it. To increase your fitness level, keep two of these the same and try to increase one. Continue to do this for a week or so and then try using a different variable etc.. If you want to keep everything the same, add some resistance training for variation and strength.
You will gradually increase your fitness level. Continue to boost your immune system, be injury free and begin your summer fun, healthy and safe!
Even in distancing we are together!