Congratulations, you’ve made the same error as 99% of us, me included! Welcome to the club. Your heart and lungs have loads of fitness from cycling and to be honest, if reasonably strong cyclist could likely run decent 5k straight off … BUT your legs/muscles/tendons etc are NOT fit for running yet.
Most cyclists jump in and do too much too soon (since it seems SO easy due to bike fitness) but take time, build up slow, do walk/run to start and it will quickly improve.
I was a cyclist for about 5 years and got reasonably decent (fairly strong club cyclist, mid table TTer) but went out and did 44 min 10k in local race off no training…then couldn’t walk for a week!
Started run training and started duathlons, got down to a low 18 min 5k 38 min 10k and managed to qualify for world sprint duathlon champs for GB age group so you never know!
That was last year and now training for my first ever marathon.
It will come…and once muscles get used to it, then cycle fitness will mean big fast improvements… BUT give yourself 4-6 weeks to slowly build up muscles/tendons etc.