Patience. I'm not a doctor but 15 years of no excersize may just be the cause.
I see so many people try too hard in the beginning and injure themselves or decide they just can't do it.
Its like the couch to 5k in 5 weeks which might be ok for someone in their twenties but won't work for everyone.
And 4 days is not long in comparison to 15 years. If it took 15 years to get this unfit it will take time to get fit again. The body has to loosen up first. Your body needs time to learn and adjust. And it's not just your legs but your back and arms and neck that need to adjust.
Make sure the bike fits and use the correct gear ratio. You should just be turning the pedals almost effortlessly not pushing on them or hopping up and down on the saddle.
Find a flat road with no wind and go up and down it in different gears till you find one that is comfortable and use that as your baseline.
And lastly be gentle on your self. Don't over complicate it. Lots of short gentle rides.
And good luck.