New to insulin

Hi. I have just injected Humilin I for the first time. My DNS has given me good advice I have been advised to inject in fatty area on my thighs, bum or arms. Trouble is I walk several miles a day and have lean thighs, bum and not much on my arms stomach is a no go area where I do have fat. I am 67 years of age and been a well controlled type 2 for 23 years.
I chose my bum to inject and had trouble with an hernia reaching. Nervous on injecting into lean thighs and any advice is appreciated.