As a fellow squash player who also occasionally runs
a) You're probably just setting off too fast. Most people who don't regularly run don't realise how painfully slowly you need to go to conserve energy. Don't run like you're on a squash court. try and run so slowly you feel like you're basically walking. I have this problem, I set off like I'm Roger Bannister and after a mile I'm knackered and I've got a stitch.
b) running is ****ing boring compared to something like squash. With squash you will have adrenaline flowing from the competition and so you will barely notice how tired you are. With running there is like nothing else to think about, so you will feel every bloody step. It takes willpower to keep going, as compared to squash its painful, boring, tiring and largely pointless.