How dangerous can a untreated spider bite be?

Minimal. Black widow and brown recluse will make you run for treatment. Common spider bites are best kept clean. In the unlikely event that marked pain, marked swelling, or marked redness develop, there's probably a secondary bacterial infection and physician's help should be sought. Extreme pain warns of the new staph; emergency help.
Absolutely. And congratulations on a procedure that may very well have saved your life -- thanks to your own wise decision to be screened and take a proactive approach to your health. And now that you're asking about fitness, i'm very glad -- you're a model of proactive self-care. Congratulations.