Speaking as someone who has come back to the game after a while and started mid world and having had a brief glance at your airline I would say this.
Fly Closer - If there is one thing I wouldn't of done with that Philadelphia is fly to Portland and Seattle. Closer routes get a lot more money, for my money you should be maximising route sectors per day. I have Fairchild Metros in GW4 which are 275kts Turboprops, are they are great. I fly them up to 460nm taking the range hit and I'm getting 4-5 sectors per day. My Emb110C's are much slower and are utilised 450nm to 670nm and I will only get two sectors out of a similarly pax carrying aircraft, yes they earn money but not as much, although they are still bedding in as far as I'm concerned. Also if I was flying them on the Fairchild routes I'd probably loose a sector (or two) per day as they are much slower. Start close and work out, work down the list carefully, you might miss a good route if you are not careful.
Find your niche - I.e. Find plenty of routes that are unserved or underserved. I've found competing with other competitors who are well established very hard and it gave me some small margins and some scary losses, although I had plenty of cash in reserve. I had some BAC1-11's in GW4 losing me hand over fist initially when I set up the routes, now they have settled they are great. Expect losses when you are competing against someone who is already established and maxed out demand/supply. What I've found is routes that are underserved or empty is they can be ramped up to good profits very quickly. After a while you can start to slowly gain market share (and profits) on bigger heavy competition routes. I'll repeat my comment, expect losses.
One thing I would say when competing with the "establishment". Frequency can be your best friend. Smaller aircraft flying more frequently seems to be a good way at eating into market share as a smaller competitor. E.g. A route with 300 pax demand. A competitor flys 2x A320's per day meaning approx 300 pax. Therefore I would fly something around the 70-100 mark. Talking Fokker 70, BAe146 etc. That will fill up to 70-90%LF much quicker that an equivalent A320. You will make money much quicker, later on you might be able to add another frequency and take market share or you could just maximise prices. Also adding another frequency is simple as you already have the RI, it will make money instantly.
The other advantage of this strategy so far as I'm concerned is that a 70-90 seat aircraft can flying other 70-90 seat routes that are not being served or have enough spare pax to fill your plane. Therefore giving you a balance between routes you have a monopoly on or are underserved and routes you want to compete on. Although I would get my airline steady before I go after competition routes. For me I would go 2x monopoly/underserved and 1x competing routes per day for a 3 sector day.
My thoughts when starting recently was to go in a find a unique fit for my airline, don't fight fire with fire, think nimbly, that is work with what you've got and then work to what you want. I also found taking it slow for the first few game months and working out slowly what I can get away with, don't spend all your cash on leases initially it doesn't give a lot of wiggle room when it comes to the small margins and potential early losses you might encounter as a mid game start up.
What I do for start pricing, usually about 4 clicks down on the -10% button. Maybe a few more if I've got heavy competition. Anything lower than that doesn't actually yield that much of an improvement in LF on new routes. Although you will get a feel for what works with that airline and GW after a while. It's the rule of thumb I use and thought you might appreciate the guideline to work to. I'd be interested to know what other people do. Keep checking back. Any route 90%+ for me gets a +10%. 70-80% I adjust if I can be bothered.
Finally - Read around, there is tonnes of good advice and stuff on this forum. Curse's guide is good from what I remember. Learn it now and you will be in a great place to take advantage of the next Game World. From what I remember starting mid game is different when compared to an early game start.