**Edit: The following is incorrect, see below.** It might be more useful to think of $\Omega_f(M)$ as sitting in a homotopy pullback
$\require{AMScd}$
\begin{CD}
    \Omega_f(M) @>>> M\\
    @V  V V @VVfV\\
   M  @>>\operatorname{Id}> M 
\end{CD} 

Then you can make use of the "Mayer-Vietoris sequence" of homotopy groups
$$\cdots \to \pi_2(M, x_0) \stackrel{\partial}{\to} \pi_1(\Omega_f(M),x_0) \to \pi_1(M, x_0) \times \pi_1(M, x_0) \to \pi_1(M,x_0) \to \cdots$$
described in [the answers here.][1] The last map above is $(a,b)\mapsto a\cdot f_*(b)^{-1}$, which is not a homomorphism in general but is a surjection of pointed sets. This immmediately shows for example that when $\pi_2(M,x_0)=0$ then $\pi_1(\Omega_f(M),x_0)$ is isomorphic to the graph of the homomorphism $f_*:\pi_1(M,x_0)\to \pi_1(M,x_0)$.

To go further you'd have to analyse the connecting map $\partial: \pi_2(M,x_0)\to \pi_1(\Omega_f(M),x_0)$. It may be (though I'm not sure) that the map preceding it is given by $(x,y)\mapsto x-f_*(y)$, in which case $\partial=0$. A good place to read up on this sequence seems to be May and Ponto's *More concise algebraic topology*, Section 2.2 

**Edit:** as Tyrone points out, I wrote down the wrong pullback square, and consequently my conclusions were incorrect. What I should have written down was the topological pullback

$\require{AMScd}$
\begin{CD}
    \Omega_f(M) @>>> C^\infty(\mathbb{R},M)\\
    @V  V V @VV\operatorname{ev}_{0,1}V\\
   M  @>>(\operatorname{Id},f)> M\times M 
\end{CD} 
 in which the map $\operatorname{ev}_{0,1}$ is homotopically equivalent to the diagonal map $\triangle: M\to M\times M$. Then the MV sequence looks like

$$\cdots \to \pi_2(M \times M) \stackrel{\partial}{\to} \pi_1(\Omega_f(M)) \to \pi_1(M \times M) \to \pi_1(M\times M) \to \cdots$$

where this time the last map is given by $(a,b)\mapsto(a b^{-1}, f_*(a) b^{-1})$. Then in the case where $\pi_2(M)=0$, for example, we can see that $\pi_1(\Omega_f(M))$ is isomorphic to the subgroup $\{a\in \pi_1(M) \mid f_*(a)=a\}$.


  [1]: https://mathoverflow.net/q/132339/8103