Tone is an ambiguous word with no clear definition. It comes from "tonus" which is the tension in muscle at rest. That's not something you can change unless a muscle is atrophied. Generally tonus has to do with age only and is something that muscles lose as people age.
Because "tone" is a slang term which means different things to different people, it is best to avoid using it.
For more, go here --> http://scoobysworkshop.com/toning/
Yoga is not one thing. It's an ancient practice taught in a hundred different ways which some believe offer health benefits while other believe it a archaic way to waste time. For example, read the following excerpt from this article --> http://en.wikipedia.org/wiki/Yoga
"Yoga has been criticised for being potentially dangerous and being a cause for a range of serious medical conditions including thoracic outlet syndrome, degenerative arthritis of the cervical spine, spinal stenosis, retinal tears, damage to the common fibular nerve, so called "Yoga foot drop," etc. An exposé of these problems by William Broad published in January, 2012 in The New York Times Magazine resulted in controversy and denial in the yoga community in New York City where yoga is popular. Broad, a science writer, experienced yoga practitioner, and author of The Science of Yoga: The Risks and the Rewards, had suffered a back injury while performing a yoga exercise. Torn muscles, knee injuries, and headaches are common ailments which may result from yoga practice.
An extensive survey of yoga practitioners in Australia showed that about 20% had suffered some physical injury while practicing yoga. In the previous 12 months 4.6% of the respondents had suffered an injury producing prolonged pain or requiring medical treatment. Headstands, shoulder stands, lotus and half lotus (seated cross-legged position), forward bends, backward bends, and handstands produced the greatest number of injuries."
In general, the value of Yoga is less a matter of fact and more a matter of how much you believe it benefits you given the way that you practice it. No one can say if Yoga is one thing or another because it is so many things.
In general, it is probably better to study, learn, and practice specific disciplines for specific reasons. For example, if you need more endurance, work on nutrition and endurance building exercises such as aerobics. If you need peace of mind, study meditation or relaxation therapy and practice them. If you need strength, study strength training and join a gym and practice. And, you probably don't need more flexibility because the most recent science suggest stretching not only does nothing good but can cause fitness problems. So, it is probably a bad idea to practice a centuries old cultural tradition with the idea that it you will reap benefits when you are not addressing specific issues with specific modalities.
In short, Yoga in the West has long been a faddist's alternative practice of questionable value and will eventually discontinued for lack of benefits by most who try it.
You should make a list of fitness goals, put them in order of priority, and study so you know enough to be able to develop a regimen which will accomplish as many of the goals as possible. What you're doing seems to be a trial and error approach to fitness which is likely to produce poor result with high risk.
Good luck and good health!!