As a rule of thumb, the best way to improve your riding is to ride. However, there are other things which could/should be used to suppliment your riding or training. I tend to think of it as the more you ride / train the more of these you should do to help recovery or flexibility.
When I started riding I would ride and that would be it, no extras. Over the few years I've been increasing my speed and distance I've added extra bits after riding which at the moment amount to basic leg stretches and more recently the use of a foam roller to massage muscles after riding and ease aches / help recovery. I also make sure to keep properly fuelled on my rides and have something nutritious to eat afterwards along with a big glass of milk because I can't stand recovery drinks nor do I really 'buy into' them.
It comes down to personal choice for how much you feel you need to be doing in terms of 'work off the bike'. I find that thoroughly stretching my quads, calves, glutes and hamstrings regularly as well as foam roling them when they ache in the hours/days after training is enough for me, but I might experiment with some core strengthening for example as I am getting uncomfortable on my longer rides sometimes.
The British Cycling Insight Zone seems like a good resource, you can have a read and decide which bits you'd like to try. Essentially, riding will be what improves you but these extras can help recovery and keep you fresher/more comfortable.
