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Hi, I wouldn't recommend doing this, it's going to be a lot of work and you probably won't see any benefit. The historical value of those URLs is much stronger than any boost you'd get by taking out a few date folders. It's very common for blogs to have this URL structure, search engines are well aware of this structure and aren't going to ding you for having long or deep folder URLs. Additionally, since your blog is 10 years old, you've probably got tons of posts, which means tons of 301 redirects. You have to take Google with a grain of salt when they come out and say things like "301s pass all authority now", it doesn't mean you should go out and redirect every page on your site. At some point you're going to begin affecting page speed, particularly on mobile devices.

there always a lot of different opinions Yes, indeed.   There will always be lots of opinions. If you hire a person to review what you do then you will get ONE opinion.  In my opinion, a lot of the opinions posted online either in articles or in forums are flawed (including a lot of what I post).  Nobody knows everything and a lot of people sell SEO services who should not be selling SEO services.  Be careful. My method is to depend mainly on myself, then when I am going to do something really important, I get a paid opinion.  I don't use forums as  a substitute for opinions on things that are more than a simple one-topic question/answer.  People who are posting in forums generally are not going to spend hours doing a deep review your site and making complex decisions.   You will not get good advice on a topic that requires a deep and detailed knowledge of your website and your business.  People posting in a forum generally don't have that kind of extra time and we should not expect that type of service in a forum.    I also believe that this type of information should remain private and not be discussed in the open in a forum. How to find someone?   My suggestion is to look at a Q&A forum like Moz daily and read lots of the questions.  After doing that for a several months you will learn a lot.  You will also see the range of people who give answers.  Some might not seem like a person you would want giving you advice, others might be much more appealing because of their ability to explain clearly, their generosity and how you perceive their knowledge of what they are talking about. Every person who has given me advice or done SEO or technical work for my website me is a person who I have met in an SEO forum.  I go to different people for different types of advice and I often get two opinions on important projects.  I am not telling you this because I am looking for work.  I never give private consultations or do SEO work for anyone.  I also never recommend people to work on the websites of others.  These are things that you should decide for yourself. Get educated about SEO.  Learn who gives good advice.

Glinski, You should not be insecure about changes to your content. Remember content is designed for the target audience and if one page is geared towards muscle size and the next is muscle gain then the content of those pages should be written to reflect those ideas. This does not mean you need to be exact with the keywords. The meaning of the content needs to reflect what you want to rank for. So if both pages have similar content google will show the best content that is related to the users search intent. So you have an opportunity to rank for two separate keywords by changing the content on one page to more closely match the users intent "Muscle Size" or Muscle Gain" These are 2 very different ideas and can be easily differentiated with good content.