I'm not sure what a 'senior research fellowship' involves or means to be honest, but I imagine that having a PhD will make you qualified to do research at a 'senior' level. I mean, research fellowships are given to PhD students, so a senior one is just a way of distinguishing them from that, or am I wrong?
Either way, well done on being offered the job. I am sure that they have offered you it because you have what it takes. I think any job you take is going to be stressful and I would go for it!