Distance from your city is the factor to look at, but there are other factors which will determine your strength compared to theirs. Increased squadsize makes a big difference, as can several of the military Ancient Wonders.1.What else, besides the range, determines the strength of opponents when capturing provinces?
2.Does it depend on the level of the town hall or only on the head of research?
3.Also at tournaments or in the spire?
Right now tournaments are the opposite. More squadsize techs researched will make them harder, but that is likely to change soon if the new tournament format makes it to Live, which will make the tournament fighting difficulty increase based on a more complex system as factors, such as number of researches completed, number of expansions placed, number of Ancient Wonder levels that you have.