I'm pretty athletic, I love walking & strength training. For a long time now my resting heart rate is 48. Should I be worried?

No. If you are asymptomatic and have no history of heart disease, you do not need to worry about a low heart rate - in general. I say this because certainly if it was in the 20s or 30s, it would be concerning. But it is not uncommon for young, athletic individuals to have asymptomatic low heart rates.
No. No. If you are fine without symptom (no dizziness chest pain nausea etc) then don't worry. Athletes tend to be bradycardic hr less than 60. Mine is usually 40.