Dog walker rates

A friend has a large dog she cannot handle on walks anymore and has asked me if I would be his walker 30 minutes a day, Monday - Friday. I could really use the money and am familiar with the dog, so she asked me before looking elsewhere. She said to name my price going up from $50 and to consider my time and gas to get there (it's a 5 mile, 20 minute round trip - we're in the city). I was thinking $15/walk, which would be $75/week. Is this reasonable? I tend to sell myself short, selling my wares at too low of a price (I've been told), so it's hard coming up with my own price. A side job (to turn carreer) I do pays $25/hr. Any feedback is appreciated. UPDATE: I went with $15/walk, $75/week, she said it was fair. It's not going to be for long as they'll be moving to a place with a yard in the near future. Thank you all for your responses!