I'm a professional boxer and when I do 10-15 rounds on bags, run for 10k and then bike for 40 minutes, when it comes to the bike stage, I do feel like i'm putting a strain on my legs because it's a lot of hard work in a single day. But i've learnt to stretch properly, especially in regions where joints are most vulnerable now, and later in life to things like osteoporosis.
Basically, eat oats for breakfast, a banana and protein shake too maybe, or as a snack. Then for lunch have (brown) pasta with chicken and vegetables, and then an hour and a half later, go to (kick)-boxing - you'll get the right energy for a long session. Before you start, do some high knees, skipping, side to side jogs to increase your heart rate, and then stretch all your muscles. I obviously can't post a visual of each but just think of it as anywhere you have a joint, bone, or muscle; stretch, twist or rotate it.
Never stretch cold muscles, hence the warm up skipping first. I've never had shin splints from running, or serious cramps or dead legs because i'm quite sensible about my pre-workout stretch routine. It might look a bit girly or whatever for a professional boxer to do, but I don't care. Safety first.