Ahh took me back reading through this. Business plans and bank managers. Back in 1980 Barclays refused to fund my plan. I told the manager OK fine I'll do it without you..... then he asked me to sit down again and offered me a loan

(I still had to sign over my house as security)
Business is about commitment. If you don't believe it no one else will and if you don't put in the effort it won't happen either. Once during the early days I worked 36 hours straight to get a job out and keep a customer - and bribed the staff to do it too. The first 10 years of the business I worked 12 hour days Mon-Fri and 6 hours Saturdays - and we still came close to going bust once.
Over the years I have had contact with many start up businesses. Those (like my own) that had their feet on the ground...watched their expenditure and kept costs under control mostly succeeded.
The other type that thought they could spend their way to success (big marketing budgets no revenue stream) almost without exception failed.
Also be prepared to adapt. My original plan was to sell direct. I'm an engineer not a salesman. I could make a good product but hated selling - we became subcontractors. Big company sells catalogue items and needs specials to complete the tender - we step in.
Lots of good advice there from Dave for any area of business. Read it all and do the sums. When I started out I earned half my previous salary in the first year of trading. By year 3 I was back where I started.
I'm now in the final stages of closing down a business that had a 2M turnover (turnover does not = profit!) ...but that's another story
If you want it - go for it.... but keep it real