I applied for a Schengen multiple entry visa but I got only one single entry with a period of stay of 21 days.

I'm traveling first to Austria; then I booked a flight to Paris, France. Is that possible or should I take only trains or vehicles to travel since my visa is only a single entry? Can I fly to other Schengen countries with a single entry visa?

Should I travel to the country that issued the visa? I'm going backpacking through Europe and I don't know my longest destination or my actual destinations, to be clearer.