I absolutely recommend levelling the mine, unless you only intend to play this game for a few weeks and move on.
It is a long term investment - it will pay for itself soon enough, and will give you a steady income for as long as you keep playing.
I also recommend levelling up the sparring pit too - this is also free xp for as long as you play. Some say it is bad or useless while you are working or sleeping..... well, it is possible to put in high level champs - some of these can take much more than a whole day just to gain one level - so why not knock out a good portion of that while you are working or sleeping? A champ in lvl3 sparring pit for 2 hours gains basically the same xp as one run of brutal 12-3. Put in 3 champs for 2 hours, and you just saved yourself 8 energy. Might not sound like much, but if you are going to be a long-term player, this is soon going to add up to a lot of xp gained and energy saved.
I also saw trips mentioned using 800 gems to quickly get all masteries maxed for Kael. Nothing wrong with that - but it is worth bearing in mind that if you have 130 max energy storage, then it actually works out cheaper just to buy energy with gems and keep grinding minotaur, you will also get silver for each run. The only downside is this is obviously a bit more time consuming than just buying the scrolls. But hey, why not just do your 30 daily auto-runs each day while you are sleeping. Max masteries in 5 days for no effort.