jmc

When Cheenu was a boy, he could run $15$ miles in $3$ hours and $30$ minutes $=3\times 60+30$ minutes $=210$ minutes, thus running $\frac{210}{15}=14$ minutes per mile. Now that he is an old man, he can walk $10$ miles in $4$ hours $=4\times 60$ minutes $=240$ minutes, thus walking $\frac{240}{10}=24$ minutes per mile. Therefore, it takes him $\boxed{10}$ minutes longer to walk a mile now compared to when he was a boy.