Is 10/2 wire ok for a 125 foot run?
Voltage drop is caused by the current you are actually pulling, not breaker trip rating.  Many people compute voltage drop based on trip, but that's silly - if you're actually pulling breaker trip current, you've got other problems.   And many people consider 3% a hard limit (a bit silly since the only number Code speaks of is 8%, but I think they are worried about installations where there might be three such drops consecutively). 

Anyway, if one were being silly like that, you are just within limits on that cable.  

Otherwise, you have loads of headroom and shouldn't give it any further thoght.   

You cannot up-breaker to 30A because you have 15A or 20A outlets on that cable, and those need 20A breaker protection.  However, if you fit a subpanel in this location with 15-20A breakers for the outlets,  you can then re-breaker the supply to 30A.  Assuming 24A draw (the sensible max), you would have 0-5.25% voltage drop, which is concerning, but not serious.  

If you are willing to fit a 10KVA transformer at the outbuilding, the circuit can deliver 7200W of 120/240V split-phase at 0-3.25% drop. That is 60A@120V or 30A@240V.  Nobody's gonna worry about 3.25%. 

If you fit two commonly available 15KVA transformers, you could deliver 14,400 watts, or double the above, at 0-1.64% drop  With two less-commonly-available 20KVA transformers, you could deliver 18,000 watts at 0-1.31% drop. This is most of a standard 100A (24KW) house service.